We studied the effect of bile acids on the chemically induced transformation of C3H/10T1/2 fibroblasts in vitro. Bile acids exerted marked cytotoxicity on the cells at concentrations of greater than 100 microM lithocholic acid or deoxycholic acid, greater than 150 microM chenodeoxycholic acid, and greater than 500 microM cholic acid. N-Methyl-N'-nitro-N-nitrosoguanidine (MNNG) at 13.6 microM showed remarkable cytotoxicity to the cells but produced transformed cell foci in the cultures. When the cells were pretreated with MNNG and then maintained in medium containing bile acids, they showed an increased number of transformed cell foci compared to cells treated with MNNG alone. No promotion was observed in cultures treated first with bile acids and then with MNNG. On the basis of the present data, we conclude that bile acids promote the carcinogenic process of C3H/10T1/2 cells and that such promoting activity is observed not only with secondary but also with primary bile acids.