Aflatoxicol (AFL), a metabolite of aflatoxin B1 (AFB1), is formed in vitro by liver preparations from several species including humans. A positive correlation appears to exist between the sensitivity of a species to AFB1 and the species ability to metabolize AFB1 to AFL. Conversion of AFB1 to AFL is, therefore, a questionable detoxification step. The carcinogenicity of a diastereoisomeric mixture of AFL, prepared chemically from AFB1, was compared to AFB1 by tumor incidences being determined in 4 groups of 20 weanling male F344 rats fed either a negative control diet with no aflatoxin, a positive 50-ppb AFB1 control diet, a 50-ppb AFL diet, or a 200-ppb AFL diet for 1 year and then killed at the end of the 2d year. The respective hepatocellular carcinoma incidences were 0, 40, 20, and 70%, demonstrating that AFL is carcinogenic in the rat. The data show that a diastereoisomeric mixture of AFL is one-half as carcinogenic as AFB1, and the dose response appeared nearly linear in that a fourfold increase in dose produced a 3.5-fold increase in tumor incidence. The data did not establish unequivocally that AFL is a proximate carcinogen, but metabolism of AFB1 to AFL should not be considered an efficient detoxification reaction.