The fate of aflatoxin B1 (AFB1) in the blood of various species of animals was studied in vitro. Examination of the distribution of radioactivity in blood incubated with [14C]AFB1 at 37 degrees C showed that high levels of radioactivity were associated with blood cells. The radioactivity was readily removed from the blood cells by washing with fresh plasma, indicating loose binding of AFB1 to blood cells. Most of the radioactivity in plasma was bound to protein. These results suggest that a large part of the AFB1 in blood in vivo may be carried not only by the plasma proteins but also by the blood cells. When chloroform extracts of plasma of [14C]AFB1-treated mouse, rat, duckling, and hamster blood were developed by thin-layer chromatography, high levels of radioactivity were found in both the AFB1 region and the aflatoxicol (AFL) region. Incubation of blood with nonradioactive AFB1 and AFL showed marked interconversion of AFB1 and AFL in the blood of rats, hamsters, mice, and Mongolian gerbils, but not in the blood of guinea pigs, rhesus monkeys, squirrel monkeys, or humans. Interconversion occurred in red blood cell suspensions but not in plasma, indicating that the red blood cells are responsible for AFB1-AFL interconversion in the blood.