Aflatoxin B1 and palmotoxin B0 are equitoxic to the developing chick-embryo (Gallus domesticus) whilst palmotoxin G0 is relatively non-toxic. Toxic lesions are present in the heart, liver, skeletal muscle, brain and cartilage in varying severities. The liver and skeletal muscle show fatty change and toxic myositis, respectively. Lesions in the heart, brain and cartilage are relatively mild. The endocardial cushion-like plaques at the base of the atrioventricular valves are lesions peculiar to aflatoxin B1 and palmotoxin B0-induced cardiac damage. It appears that these mycotoxins are not selectively tissue specific in inducing organ damage in the chick-embryo. An ultrastructural study of these lesions in the chick and other species may help to elucidate the molecular mechanism of the toxicity of these mycotoxins which are suspected to be very potent human hepatocarcinogens in certain parts of the tropics. Their acute phase effects in man are, however, unknown.