AbstractThe unexplained variations in tumor incidence among laboratory rodents in various institutes, or even in the same laboratory at different times, might be due to the occasional contamination of the animal diet with mycotoxins, such as the carcinogenic T‐2 toxin and related trichothecenes, or the estrogenic zearalenone or both. These substances are secondary metabolites ofFusariumand certain other species of field fungi and often contaminate cereals harvested in wet weather.