Cell Count Reagent SF allows sensitive colorimetric assays by utilizing highly water-soluble tetrazolium salt. WST-8 produces a water-soluble formazan dye upon reduction in the presence of an electron carrier. Since the absorbance at 450 nm is proportional to the number of viable cells in the medium, the viable cell number can be determined using the absorbance value of a previously prepared calibration curve. This product is a one-bottle solution; no premixing of components is required.