Fetal Bovine Serum (FBS)

Fetal bovine serum (FBS), also called fetal calf serum (FCS), is the liquid fraction of blood collected from bovine fetuses at slaughterhouses, clarified and filtered for use as a cell-culture supplement. It is added to a basal growth medium — typically at 5-10% of final volume — to supply the growth factors, hormones, attachment proteins, and trace nutrients that most defined basal media (DMEM, RPMI-1640, and similar formulations) do not fully provide on their own. FBS is the most widely used serum supplement in mammalian cell culture because fetal blood carries a very low antibody/immunoglobulin load (the fetal immune system has not yet matured) and a comparatively high concentration of growth-promoting proteins, both of which make it more broadly supportive of in-vitro cell growth than serum collected from adult or newborn animals.

For a procurement officer, lab manager, or new PI setting up a cell-culture program, “what is FBS” is really two questions: what the product biologically is, and what has to be documented and verified before a specific lot is fit to buy and use. This page covers both.

What is in fetal bovine serum

FBS is a complex, undefined biological fluid rather than a single characterized compound, which is precisely why sourcing and lot documentation matter so much when purchasing it. Its composition typically includes:

• Growth factors and hormones — including insulin-like growth factors, transforming growth factor, and other mitogenic proteins that drive cell attachment and proliferation.
• Albumin and other transport proteins — which bind and carry lipids, hormones, and trace minerals to cells in culture.
• Amino acids, lipids, and carbohydrates — supplementary nutrients beyond what the basal medium supplies.
• Trace elements and vitamins at low, variable concentrations.
• A low but non-zero immunoglobulin and hormone background, which is why FBS is generally unsuitable, without additional processing (e.g., charcoal-stripping to remove steroid hormones), for endocrine-sensitive assays.

Because these components come from a biological source rather than a manufactured formulation, concentrations vary lot to lot, season to season, and herd to herd — the reason cell-culture labs qualify individual lots against their own cell lines rather than assuming any two bottles of FBS from the same supplier perform identically (see “What to evaluate before purchasing” below).

Fetal bovine serum vs. fetal calf serum: is there a difference?

No — “fetal bovine serum” and “fetal calf serum” refer to the same product. FBS is the term more commonly used in North America; FCS is more common in Europe and in older literature. Both describe serum collected from bovine (cattle) fetuses, as distinct from:

• Newborn calf serum (NCS) — collected from calves shortly after birth, not from fetuses. It has a different growth-factor and protein profile and is generally lower-performing for demanding cell lines, but is also less expensive and is used for applications with lower serum-quality requirements.
• Adult bovine serum (ABS) — collected from mature cattle, with the lowest growth-factor content and highest immunoglobulin load of the three; used mainly where cost matters more than growth performance.

“Bovine fetal serum” is simply a word-order variant of the same term and is not a distinct product.

Fetal bovine serum in cell culture

FBS is used across most areas of mammalian cell culture where a defined, serum-free formulation either does not yet exist for the cell type in question or has not been validated for the lab’s specific application: primary cell isolation, established cell-line maintenance (e.g., HEK293, CHO, HeLa), hybridoma and monoclonal antibody production, and stem-cell and primary-tissue culture. It functions as a supplement, not a complete medium — cells are still grown in a basal medium (DMEM, RPMI-1640, F-12, or others) with FBS added, along with antibiotics and a buffering system, per standard cell-culture protocol. CASRAI’s Cell Culture Basics guide covers how serum fits into a complete culture-medium setup for labs new to the technique, and the Cell Culture Reference Numbers guide gives standard media-volume and seeding-density figures for common vessel formats.

What to evaluate before purchasing fetal bovine serum

Because FBS is an animal-derived, biologically variable material used in regulated and research-critical workflows, procurement decisions should be based on documented, verifiable criteria rather than price alone. The dimensions that matter most:

• Country/region of origin and disease-risk status. Sourcing region affects transmissible spongiform encephalopathy (TSE/BSE) risk classification. Many labs, and most regulated manufacturing programs, specify serum sourced from countries with a recognized negligible or controlled BSE-risk classification (commonly Australia, New Zealand, and parts of the United States and South America), and require the supplier to document country of origin and collection site on the Certificate of Origin. Importing bovine-derived biological material into the US is also subject to USDA APHIS controls — see CASRAI’s USDA APHIS Import Permit guide for how those permit categories work.
• Traceability and closed-herd sourcing. A defensible supplier can trace a given lot back to a specific herd, collection facility, and collection date — not just a country. This matters both for BSE-risk documentation and for root-causing a lot that underperforms after it’s already in use.
• Testing panel and Certificate of Analysis (CoA). A usable CoA should document, at minimum: sterility (bacterial, fungal, mycoplasma), a bovine viral testing panel (commonly including bovine viral diarrhea virus and other agents relevant to bovine-sourced biologics), endotoxin level, hemoglobin level (a proxy for hemolysis and overall serum quality), and total protein/IgG concentration. Ask for the actual CoA for the specific lot being quoted, not a generic specification sheet.
• Irradiation status. Gamma-irradiated FBS has been treated to inactivate a broad range of viral and other adventitious agents and is standard for regulated or GMP-adjacent work; non-irradiated FBS is sometimes preferred for basic research where irradiation-induced changes to serum proteins are a concern for a specific assay. Confirm which the quote is for — the two are not interchangeable for every use case.
• Heat inactivation. Heat-inactivated FBS (typically 56°C for 30 minutes) has had complement proteins denatured, which some applications (e.g., certain immunology and virology assays where active complement would interfere) require and others don’t need. This can usually be specified at order time or done in-house after delivery.
• Lot-to-lot consistency data and lot reservation. Because composition varies by lot, most cell-culture labs qualify a candidate lot against their own critical cell lines (a growth-promotion or plating-efficiency test) before committing to it, then reserve enough of that specific lot to cover an extended run of work — sometimes months to years for a line sensitive to serum variability — rather than buying whatever lot is currently in stock each time.
• Cold-chain and shipping documentation. FBS is shipped frozen or refrigerated and is sensitive to freeze-thaw cycling and temperature excursions in transit; a supplier should be able to provide shipping-temperature confirmation for the specific shipment, not just a general claim of cold-chain capability.
• Relevant standards and regulatory alignment. For therapeutic and advanced-therapy manufacturing specifically, ancillary materials including serum are increasingly expected to be sourced and documented against ISO 20399, the international standard covering ancillary materials used in the production of cellular and gene-engineered products. Even outside regulated manufacturing, aligning a purchasing spec to that standard’s documentation categories (origin, testing, traceability) is a reasonable baseline for a research lab evaluating suppliers.

None of this requires favoring one named commercial supplier over another — the criteria above are what to ask any supplier to document, and they’re the basis on which quotes should actually be compared, rather than list price alone.

Worked example: qualifying a new FBS lot

Illustrative composite, not a real institution or trial: a cell-culture core facility that relies on a serum-sensitive hybridoma line receives a quote for a new FBS lot after its previous reserved lot runs low. Before switching, the facility requests the CoA for that specific lot number (not a general spec sheet), confirms country-of-origin and irradiation status match its standing SOP, and runs a small-scale growth-promotion comparison against its current lot using its own hybridoma line before placing a full order — then reserves enough of the new lot, once qualified, to avoid re-qualifying again for as long as reasonably possible. This lot-qualification-before-commitment pattern, not brand loyalty to a particular supplier, is the standard risk-management practice cell-culture labs use to avoid an unplanned serum-related failure in an ongoing line.

What isn’t fetal bovine serum

Newborn calf serum and adult bovine serum (described above) are related but distinct products, collected postnatally rather than from a fetus, and generally used where cost matters more than maximum growth performance. Serum-free and chemically defined media — increasingly available for many common cell lines — are a different category entirely: fully synthetic or recombinant-protein-based formulations designed to remove animal-serum variability, BSE/TSE risk, and lot-to-lot inconsistency altogether. Human platelet lysate (hPL) is another serum alternative, derived from human platelets rather than bovine fetuses, used mainly in cell-therapy manufacturing where a xenogeneic (animal-derived) component is undesirable. None of these are fetal bovine serum, though they are frequently evaluated as substitutes for it during procurement or process-development decisions.

Related terms

See also CASRAI’s guides on cell culture basics, cell cryopreservation, and the USDA APHIS import permit process for animal-derived biological materials, as well as the dictionary entry on Good Clinical Laboratory Practice (GCLP) for the broader quality-documentation context regulated labs work within.

Frequently asked questions

What is fetal bovine serum used for?

It’s used as a growth-supporting supplement added to basal cell-culture media, supplying growth factors, hormones, and attachment proteins that support the growth of primary cells, established cell lines, and hybridomas in vitro.

What is in fetal bovine serum?

Growth factors, hormones, transport proteins (including albumin), amino acids, lipids, carbohydrates, and trace elements — an undefined mixture whose exact concentrations vary by lot, which is why lot qualification is standard practice before adopting a new lot.

Is fetal bovine serum the same as fetal calf serum?

Yes. FBS and FCS are the same product; the terminology difference is regional (FBS more common in North America, FCS in Europe), not a difference in what the product is.

Why is fetal bovine serum used instead of adult bovine serum?

Fetal serum has a substantially lower antibody/immunoglobulin load and higher concentration of growth-promoting proteins than serum from newborn or adult cattle, making it more broadly supportive of cell growth across a wider range of cell types and applications.

What should a lab check before buying a new lot of FBS?

At minimum: the lot-specific Certificate of Analysis (sterility, viral testing panel, endotoxin, hemoglobin), country of origin and BSE-risk classification, irradiation status, and — for any serum-sensitive line — a small-scale growth-promotion test against the lab’s own cells before committing to a full purchase.