A highly water soluble disulfonated tetrazolium salt, 4-[3-(2-methoxy-4-nitrophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3-benzene disulfonate sodium salt, was synthesized. The compound 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. The tetrazolium salt thus proved to be useful as a sensitive chromogenic indicator for NADH. It is also applicable to cell proliferation assays as a cell viability indicator.