The reported LD50 for the adult, male Fisher rat is 1.2 mg aflatoxin B1/kg body weight (i.p.); we have found that male C57BL/6 mice survive single doses of aflatoxin B1 as high as 60 mg/kg (i.p.). We have demonstrated a 1000-fold greater LC50 of aflatoxin B1 for primary mouse liver cell cultures from C57BL/6 male mice (3 X 10(-5) M) than for primary liver cells from F344 male rats (3 X 10(-8) M). In 4 h of exposure to a non-toxic dose (1 X 10(-9) M) of [3H]aflatoxin B1, cultured rat liver cells accumulated up to 5-fold higher concentrations of 3H label than did mouse liver cells. The difference in cell-associated counts was due largely to higher levels of aflatoxin B1 metabolites bound to macromolecules in the rat cells.