A new tetrazolium compound, sodium salt of 4-[3-(4-iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1, 3-benzene disulfonate (1a), which produces a highly water-soluble formazan dye, due to the presence of two sulfonate groups, was synthesized and its potential utility evaluated in assays of NADH and cell proliferation. The compound proved to have a similar sensitivity to 2, 3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium hydroxide (XTT) in the assay of NADH and was also useful as an indicator of cell viability, with less cytotoxicity than XTT, in the proliferation assay using P388 cell lines.