1. Using dog liver microsomal preparations, we have isolated and identified a new metabolite of aflatoxicol, aflatoxicol M1; this metabolic transformation is analogous to the conversion of aflatoxin B1 to aflatoxin M1. 2. Aflatoxicol M1 can also be produced from aflatoxin M1 by a reductase present in rabbit liver cytosol, a reaction analogous to the reduction of aflatoxin B1 to aflatoxicol. 3. In addition, aflatoxicol M1 can be oxidized to aflatoxin M1 by a carbon monoxide-insensitive dehydrogenase activity associated with human liver microsomes, a reaction analogous to the production of aflatoxin B1 from aflatoxicol by this fraction. 4. Ultra-violet, fluorescence, and mass spectral characteristics of aflatoxicol M1 are described. 5. The significance of this new aflatoxin metabolite and the possibility that it may be biologically active are discussed.