Maize culture material of 25 isolates of the genera Rhizopus and Mucor caused deaths in day-old unsexed Pekin ducklings when fed as a 50% (w/w) mixture with duckling feed. Nine of these isolates were tested for toxicity in young inbred male BD IX rats, which were fed a mixture of 50% (w/w) culture material and rat feed. Only one isolate of Rhizopus microsporus was clearly toxic, causing 100% mortality in rats within 10 days. Growth in rats was reduced by adding culture material of this isolate to the feed in concentrations of 2.5, 5, 10 or 20% (w/w). The same isolate of R. microsporus was used to produce the mycotoxin rhizonin A. Pure rhizonin A was dissolved in dimethylsulphoxide and given to young male partially inbred albino rats by gavage in single doses of 70, 96, 131 or 180 mg/kg. The lowest dose exceeded the LD100. Evaluated by light microscopy, lesions in livers and kidneys were similar in rats fed culture material and in those intubated with rhizonin A. Hepatocytes showed changes ranging from degeneration to necrosis, the liver-tissue architecture was changed by disassociation of liver cell cords and there was periportal bile-duct proliferation. Renal tubular epithelium showed changes ranging from degeneration to necrosis.