Various food products of plant origin were investigated for the occurrence of natural mutagens using the Salmonella/microsome assay. In general, food plants were freeze-dried and subsequently extracted with a number of solvents. Solvents were evaporated and the residues obtained were tested for mutagenicity. In addition to S9-mix, gut flora extracts were applied for metabolic activation. From bracken fern (Pteridium aquilinum) a novel mutagen, designated Aquilide A, was isolated and its chemical structure was identified. Aquilide A requires activation to become mutagenic. This activation occurs spontaneously at pH levels above 6-7. Activated Aquilide A was found to be genotoxic in cultured mammalian cells. Natural mutagens were detected in 4 out of 6 vegetables investigated. In addition, broad beans (Vicia faba) were found to be mutagenic after treatment with nitrite. All mutagenic vegetables showed marked intercultivar variations. From lettuce and string beans quercetin was isolated (after chemical hydrolysis) and in rhubarb emodin, an anthraquinon, was detected. The mutagenic activity of these two compounds was further investigated using cultured mammalian cells. Quercetin and emodin responded negative or weakly positive in the systems applied. The genotoxic properties of a number of pyrrolizidin alkaloids, which are reported to occur in various flowering plants and as a result occur in honey and some herbal preparations, were studied using a cocultivation system of V79 Chinese hamster cells and primary cultures of chick embryo hepatocytes (PCCEH/V79). All four pyrrolizidine alkaloids investigated were found to be potent inducers of SCEs in this test system. Anti-mutagenic effects of butylated hydroxyanisole (BHA) and indole-3-carbinol (I3C) were detected using the PCCEH/V79 cocultivation system. This indicates that the cocultivation system described can be a valuable tool for the screening of various products for potential anti-carcinogenic properties. Extracts of lettuce and string beans, and a number of natural chemicals were found to reduce the mutagenic activity of cigarette smoke condensate and benzo (a) pyrene (BaP) as detected in the Salmonella/microsome assay. Intercultivar variation with respect to the antimutagenic activity observed was less pronounced than the variation noted for the mutagenic activity of these vegetables. Measures which may result in a reduction of the exposure to a number of natural mutagens are discussed.