The uptake and binding of ring-labelled [-14C]aflatoxin B1 (AFB1) by rat and hamster liver and kidney has been studied, the former species being extremely sensitive to the carcinogenic action of AFB, whereas the latter is resistant. In contrast to an earlier report (Lijinsky et al, Cancer Res., 30 (1970) 2280-2283, binding of the carcinogen to nucleic acids was far greater than that to protein. Rat liver DNA bound ten times and rRNA twenty times more carcinogen than protein. There were also differences in the amount of carcinogen bound to rat liver nucleic acids compared to those of the hamster, the latter species binding lower amounts of the carcinogen. Rat liver DNA bound four times and rRNA ten times as much AFB1 6 h after carcinogen administration whereas liver protein bound AFB1 was similar for the two species. Not only was there a difference in the amount of AFB1 bound but whereas in the rat, liver nucleic acid bound carcinogen decayed with time, no such fall was seen in the hamster, this remaining at a low level throughout the 48-h time period studied. In contrast, reaction of the carcinogen with kidney macromolecules was similar for the two species. The much higher binding of AFB1 to nucleic acids than to protein might account for the potent carcinogenicity of this compound in the rat, particularly since liver protein binding does not differ between a susceptible and a resistant species. A further important factor in determining carcinogenic sensitivity may be the removal of nucleic acid bound radioactivity with time, a possible repair process.