Cell viability and in vitro cytotoxicity assay methods were developed using a combination of dyes, 4-[3-(4-iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1, 3-benzene disulfonate (WST-1), neutral red (NR) and crystal violet (CV), with HeLa cells as a bioindicator. As WST-1 produces a highly water soluble and non-cytotoxic formazan dye, it allows each assay to be carried out in one culture dish. The combined cell viability assay using WST-1, NR and CV gave an absorbance that correlated linearly with the number of cells over the range 1000 to 50000 cells/well. The combined assay was applied to the evaluation of the IC₅₀ values for sodium dodecyl sulfate as a model toxicant, which yielded similar values to those obtained with each assay independently.