The specificity and sensitivity of the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) tetrazolium assay can be influenced by certain factors, such as colored substances and cell volume. When the MTT assay is applied to measure cytotoxicity induced by emodin, its accuracy might be affected by emodin itself. Vascular smooth muscle cells were cultured in M199 medium. The optical density of emodin or formazan was measured by spectrophotometry. Emodin has a different absorption spectrum in different solvents. The solvents containing water induced a red shift of the absorption curve of emodin, which increased the overlap of the absorption curves of emodin and formazan. Formazan was formed from the MTT tetrazolium salt by emodin in a dose-dependent manner, which was partially suppressed by serum. Cytotoxicity was induced by emodin in a time- and dose-dependent manner in a modified MTT assay. The data suggest that emodin can alter the accuracy of the MTT assay but that a modified MTT assay is still valuable in measuring emodin-induced cytotoxicity.