The objective of this study was to assess cytotoxic effects of ochratoxin A (OTA), penicillic acid (PA), citrinin (CIT), fumonisin B1 (FB1) and their combinations (as naturally occur in real practice) on human peripheral blood mononuclear cells (PBM) using a simple and cheap “in vitro” cytotoxicity test as MTT assay. A stronger suppression on metabolic activity of phytohaemagglutinin-p (PHA)-stimulated PBM was found for a mixture of the following mycotoxins: OTA, CIT and FB1 as compared to any single mycotoxin. PA was found to increase significantly the metabolic activity of the same cells in concentrations above 111 mg/L. There is no “in vitro” synergistic effect between OTA and PA as measured by MTT assay, unlike the strong “in vitro” synergistic effect between the same mycotoxins. The MTT bioassay is not appropriate to measure cell viability and cell proliferation, especially if metabolic activity of the cells is increased as was observed in PA treated human blood mononuclear cells or if the same is decreased. The doses which exert approximately 50% suppression of metabolic activity of PHA-stimulated human blood mononuclear cells (LD50) are determined to be as follows: for OTA - 1.56 to 3.125 mg/L, CIT - 62.5 to 111 mg/L and FB1 - 62.5 to 500 mg/L. Cell viability (proliferation) of all mycotoxin treated blood mononuclear cells was significantly decreased at the highest concentrations of mycotoxins, but this decrease was significantly stronger for OTA-treated cells and especially for different mixtures of mycotoxins.