The CellTiter 96 AQueous Non-Radioactive Cell Proliferation Assay is a homogeneous, colorimetric method for determining the number of viable cells in proliferation, cytotoxicity or chemosensitivity assays. The CellTiter 96 AQueous Assay is composed of solutions of a novel tetrazolium compound [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt; MTS] and an electron coupling reagent (phenazine methosulfate) PMS. MTS is bioreduced by cells into a formazan product that is soluble in tissue culture medium. The absorbance of the formazan product at 490nm can be measured directly from 96-well assay plates without additional processing. The conversion of MTS into the aqueous soluble formazan product is accomplished by dehydrogenase enzymes found in metabolically active cells. The quantity of formazan product as measured by the amount of 490nm absorbance is directly proportional to the number of living cells in culture. If you currently use a [(3)H]-thymidine incorporation assay, addition of the combined MTS/PMS solution can be substituted for [(3)H]-thymidine at the time point in the assay when the pulse of radioactive thymidine is usually added. Data from proliferation bioassays comparing the CellTiter 96 AQueous Assay and [(3)H]-thymidine incorporation show similar results. This is in agreement with similar radioactivity incorporation studies performed using the original CellTiter 96 Assay. CellTiter 96 AQueous MTS Reagent Powder is a novel tetrazolium compound for use in colorimetric assays for determining the number of viable cells in proliferation, cytotoxicity or chemosensitivity assays. It is provided in powdered form.