The MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay is a widely used screening method to measure cell viability and proliferation. When testing the effects of kaempferol on breast cancer cell number (crystal violet staining) and viability (MTT tetrazolium assay) conflicting results were obtained. Cell number decreased but MTT formazan formation increased, suggesting a direct interaction of kaempferol with the MTT tetrazolium reduction. Direct reductive potential was observed in a cell-free system for the presumptive phytoestrogens kaempferol and resveratrol, and extracts of Hypericum perforatum L. and Cimicifuga racemosa L. All agents led to instantaneous dark blue formazan formation in the absence of cells. Additionally, antioxidants such as ascorbic acid, vitamin E and N-acetylcysteine interfered with the MTT tetrazolium assay. When MCF7 and HS578 cells treated with kaempferol were washed before addition of MTT tetrazolium, the direct reduction of dye was reduced significantly. These results indicate that the MTT tetrazolium assay may lead to false positive results when testing natural compounds with intrinsic reductive potential.