The tetrazolium salt 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) is used to determine cell viability in assays of cell proliferation and cytotoxicity. MTT is reduced in metabolically active cells to yield an insoluble purple formazon product. However, in the present study, we demonstrate that some botanical extracts can reduce MTT in the absence of living cells. Treatment of these extracts with iodoacetic acid (IAA) to alkylate free thiol groups inhibited their ability to reduce MTT, indicating that free thiols in the botanical extracts are responsible, at least in part, for the reduction of MTT to the formazon product. These results suggest that caution must be taken when interpreting the results of experiments using the MTT assay to measure the effects of botanical extracts on cell growth.