The ready-to-use cell proliferation reagent, WST-1 provides a simple and accurate method to measure cell proliferation, which is based on the cleavage of the tetrazolium salt WST-1 to formazan by cellular mitochondrial dehydrogenases. Expansion in the number of viable cells results in an increase in the activity of the mitochondrial dehydrogenases, which in turn leads to increase in the amount of formazan dye formed. The formazan dye produced by viable cells can be quantified by measuring the absorbance at 440 nm. This new method is non-radioactive, rapid and more sensitive than MTT, XTT, or MTS-based assays. The entire assay can be performed in the same microtiter plate and does not require extra steps like washing, harvesting and cell solubilization.