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Short answer. CellTiter-Glo is a Promega luminescent cell viability assay that measures ATP, a marker of metabolically active cells. The reagent is added directly to cells in culture medium, lyses them, and the released ATP drives a luciferase reaction whose light output is read on a luminometer or plate reader (Promega product page, read 2026-11-07).
Definition and scope
“CellTiter-Glo” (also written CellTiter Glo, Cell Titer Glo or CellTiter-Glo Luminescent Cell Viability Assay) is the name of a family of Promega reagents, not a generic method. All of them quantify ATP as a proxy for the number of metabolically active cells. This page covers the reagent family, how the readout works, how the variants differ and what the number can and cannot tell you. It is general laboratory information, not a validated protocol: follow the technical bulletin or manual for the kit lot you hold.
How does CellTiter-Glo work?
Per Promega’s product page, the assay is homogeneous: the reagent is added directly to cells in their medium with no washing or medium removal. The reagent lyses the cells, and the ATP released drives a reaction between luciferase and luciferin that produces a “glow” luminescent signal proportional to the amount of ATP, and therefore to the number of metabolically active cells. For the enzyme-reporter relative of this chemistry, see luciferase reporter assays.
Workflow as the manufacturer describes it
- Add reagent to the well. The product page gives 100 µl per well in 96-well plates and 25 µl per well in 384-well plates; the original quick protocol (TB288) describes adding a volume equal to the culture medium volume.
- Mix for 2 minutes on an orbital shaker to lyse the cells.
- Incubate about 10 minutes to stabilize the signal, then read. Promega states the signal half-life is generally longer than 5 hours, depending on cell type and medium.
- Opaque-walled multiwell plates are required and are user-supplied; the reading instrument is a luminometer or a plate reader with luminescence capability.
Promega reports sensitivity down to about 15 cells per well in 384-well format for the original assay, and a linear range up to five orders of magnitude. Treat these as manufacturer specifications under their test conditions, not as guarantees for your cell line.
CellTiter-Glo variants compared
The comparison below is drawn from Promega’s product page (read 2026-11-07). Every cell is the manufacturer’s statement.
| Variant | Catalog series | Format | Plate formats | Stated distinguishing point |
|---|---|---|---|---|
| CellTiter-Glo (original) | G7570-G7573 | Two components: lyophilized substrate and buffer | 96, 384, 1536 | Requires reconstitution; reconstituted reagent at 4 °C loses about 5% activity after 48 hours and about 20% after 4 days |
| CellTiter-Glo 2.0 | G9241-G9243 | Single ready-to-use solution | 96, 384, 1536 | No reconstitution; about 15 minutes equilibration; about 10 cells per well detected in 96-well format |
| CellTiter-Glo 3D | G9681-G9683 | Modified lysis chemistry | 96, 384 | Designed for dense three-dimensional cell structures |
A fourth product, CellTiter-Glo One Solution (G8461-G8462), is listed on the same page. For the 2.0 formulation, the technical manual TM403 summary states light output remains above 85% after 2 months at 4 °C and after 7 days at room temperature.
What a luminescence readout cannot tell you
This is the practical gap in catalog and protocol pages. Based on how the assay is described by the manufacturer, and offered as editorial interpretation:
- ATP is a proxy, not a count. The signal tracks ATP from metabolically active cells. A treatment that changes cellular ATP without changing cell number will shift the reading; a direct count (for example with a hemocytometer) answers a different question.
- It is an endpoint assay. The reagent lyses the cells, so the same wells cannot be followed over time.
- Viability is not mechanism. A drop in signal does not say whether cells died or stopped proliferating; see cytotoxicity and cell proliferation assays.
- Different readouts, different biology. Tetrazolium assays such as MTT, XTT and CCK-8 are not ATP-based. For the broader ATP-assay context see ATP assays.
Where it matters for labs and supply decisions
A checklist built only from the manufacturer’s stated requirements:
- A luminometer or luminescence-capable plate reader.
- Opaque-walled multiwell plates (user-supplied).
- The reagent version that matches your handling: the original requires reconstitution and has stated stability windows once reconstituted; 2.0 is supplied ready to use; 3D is for dense 3D structures.
- The plate format your throughput needs: 96/384/1536 for original and 2.0, 96/384 for 3D.
- The kit’s own safety data sheet and technical bulletin for the lot in hand.
Pricing is set by vendors and distributors and is not stated here.
Common misunderstandings
- “The signal is a cell number.” It is an ATP-derived signal that tracks cell number only under controlled conditions.
- “Any plate works.” The manufacturer specifies opaque-walled plates.
- “The reagent is the same across versions.” Handling, reconstitution and stability differ, as the table shows.
FAQ
How does CellTiter-Glo work?
It lyses cells and uses the released ATP to power a luciferase reaction that emits light, read on a luminometer; see the section above.
What is a cell proliferation assay?
A method that estimates how many cells are present or dividing, usually through a surrogate signal such as ATP or metabolic activity. See the cell proliferation assay guide. CellTiter-Glo measures ATP as a viability proxy.
What is an acceptable cell viability percentage?
The manufacturer sources used here state no universal threshold. Acceptable values depend on the assay, the cell type and the controls (untreated and no-cell wells) defined in your protocol.
How much does an MTT assay cost?
Cost depends on vendor, kit and plate format and is not stated by any source used here. The method itself is described in the MTT assay guide.
What wavelength does CellTiter-Glo read at?
Luminescence is detected as emitted light rather than absorbance at a set wavelength; consult the technical manual for instrument settings.
Sources
- Promega, CellTiter-Glo Luminescent Cell Viability Assay product page: promega.com/products/cell-health-assays/cell-viability-and-cytotoxicity-assays/celltiter-glo-luminescent-cell-viability-assay/ (read 2026-11-07)
- Promega, CellTiter-Glo 2.0 Assay Technical Manual TM403 (summary page, read 2026-11-07)
- Promega Technical Bulletin TB288, CellTiter-Glo Luminescent Cell Viability Assay (quick protocol)
This page is general laboratory information and not medical advice.
