About 100 fungal metabolites may cause cancer, embryological defects, or other histopathological effects in mammals. They arc produced by a wide variety of fungi. Few of these metabolites have significant acute toxicity. With the exception of aflatoxin B, and sterigmatocystin, there is no conclusive evidence that any of them is carcinogenic. However, several of the compounds are mutagenic. Cytochalasin D and T‐2 toxin are probably teratogenic. A wide variety of other histopathological effects have been shown. Liver damage has been most frequently reported. In almost all cases the molecular bases of these effects have not been extensively investigated. Although much is known about the routes by which some of the compounds are synthesized in vivo, nothing is known about control at the molecular level of these biosynthetic routes. Little is known about the biological degradation of these compounds or about the levels and incidences of them in food and animal feed. Future work in all these areas will depend on the further development of sensitive assay methods that are applicable to their measurement in food, in animal feed, and in animal tissues and body fluids and on the application of these methods to define exposure to these compounds in the diet.