The pesticides benomyl, a benzimidazole fungicide, and pirimiphos-methyl, an organophosphorus insecticide, were tested separately and in combination at a ratio of 6:1, a mixture frequently found in foodstuffs by residual analysis, to determine their possible genotoxic action. The effect was measured by the micronucleus test carried out on cultured rat hepatocytes stimulated to proliferate by epidermal growth factor (EGF). Adult rat hepatocytes were exposed in vitro for 48 h to the substances at increasing non-cytotoxic doses, chosen on the basis of cytotoxicity tests such as LDH and Neutral red assays. Benomyl induced a significant dose-related increase in micronucleus frequency; in contrast, pirimiphos-methyl was not genotoxic at any dose tested. When the hepatocytes were exposed to the two pesticides together at increasing doses, an enhancement in micronucleus frequency similar to that of benomyl alone was found, indicating that at this ratio and non-cytotoxic doses (up to 25 micrograms/ml benomyl + 4.2 micrograms/ml pirimiphos-methyl) no interaction occurs.