Although epidemiological studies suggest that aflatoxin B1 (AFB1) is a human carcinogen, at least in the presence of hepatitis B virus infection, animal studies have demonstrated large differences in species sensitivity to AFB1, and the sensitivity of humans relative to experimental animals remains unclear. The purpose of this study was to determine the profile of AFB1 metabolism and the extent of AFB1 binding to cell macromolecules in human liver slices under experimental conditions that would allow direct comparison to similar endpoints in the rat, a species sensitive to the carcinogenic actions of AFB1. Liver slices were prepared from three individual human liver samples with a Krumdieck tissue slicer and incubated with 0.5 microM [3H]AFB1 for 2 hr. Significant interindividual variations were observed in the rates of oxidative metabolite formation and in specific binding to cell macromolecules. The rates of oxidative metabolism of AFB1 to AFQ1, AFP1, and AFM1 in the three human liver samples were similar to those previously observed in rat liver slices. AFB1-GSH conjugate formation was not detected in any of the human liver samples, and yet specific binding of AFB1 to cell macromolecules was considerably lower in the human liver slices relative to that in rat liver slices. AFB1-DNA binding levels ranged from 3 to 26% of control rat and AFB1-RNA binding levels ranged from 25 to 49% of control rat. The AFB1-protein binding level in the one human sample measured was 20% of that observed for control rat. While these results suggest that humans do not form as much AFBO as the rat, they are also consistent with the hypothesis that humans do not possess GST isozyme(s) with high specific activity toward AFBO. Significant individual differences in AFB1 metabolism and binding between humans suggest the presence of genetic and/or environmental factors that may confer large variability in susceptibility to AFB1.