WST-8 is reduced by NADH in good yields at neutral pHs in the presence of 1-methoxy PMS to produce the corresponding formazan dye that absorbs at 460 nm. The formazan is soluble to water at concentrations higher than 0.1 M. WST-8 is useful as a sensitive chromogenic indicator for NADH. It is also applicable to cell proliferation assays as a cell viability indicator.