The CellTiter 96® AQueous One Solution Cell Proliferation Assay is a colorimetric method to determine the number of viable cells in proliferation or cytotoxicity assays. The CellTiter 96® AQueous One Solution Reagent contains a novel tetrazolium compound [3-(4,5-dimethyl-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt; MTS] and an electron coupling reagent (phenazine ethosulfate; PES). PES has enhanced chemical stability, which allows it to be combined with MTS to form a stable solution. This convenient "One Solution" format is an improvement over the first version of the CellTiter 96® AQueous Assay, where phenazine methosulfate (PMS) is used as the electron coupling reagent, and PMS Solution and MTS Solution are supplied separately.