Aflatoxicol, a carcinogenic metabolite of the foodborne carcinogen aflatoxin B1 previously known only as a bioreduction product in vitro, was identified as the major aflatoxin metabolite in the plasma of Sprague-Dawley rats, a susceptible species, that had been doses orally or intravenously with aflatoxin B1 labeled with carbon-14. Alfatoxicol, however, was not detected in the plasma of similarly dosed mice and monkeys, which are both resistant to aflatoxin B1-induced cardinogenesis. The formation of aflatoxicol both in vitro and in vivo may be an indicatory of species sensitivity to aflatoxin-induced carcinogenesis and may be useful in the prediction of human susceptibility.