The cocarcinogenic effect of bile acids on the chemical transformation of C3H/10T1/2 fibroblasts was examined in vitro. The assimilation of 3H-methylcholanthrene (3H-MCA) by C3H/10T1/2 cells pretreated with bile acids was examined by the measurement of uptake and by autoradiography. Cells that were pretreated for 48 h with bile acids (100 μM lithocholic acid, 500 μM cholic acid) and then maintained in medium that contained 3H-MCA showed an increase in radioactivity compared to control cells. These results indicate that the transfer of carcinogens into cells is enhanced by pretreatment with bile acid