The promoting effect of sodium deoxycholate (DC) on colon carcinogenesis was studied in female F344 germfree rats. Animals received intrarectaI (ir) instillations of N-methyl-N′- nitro-N-nitrosoguanidine (MNNG) for 4 weeks (total dose, 16 mg/rat), then weekly ir doses of DC (total dose, 3 g/rat); the rats were autopsied 52 weeks after the first injection. DC increased the number ofMNNG-induced colon adenocarcinomas. No tumors were in the colons of germfree rats given DC alone. It was concluded that DC (present in high concentrations in human stools) had a promoting effect on colon carcinogenesis in rats.