AbstractThe effect of feeding skim milk fermented byStreptococcus thermophilusorLactobacillus bulgaricuson the incidence of chemically induced colon tumors was studied in rats. Weanling Fisher‐344 rats were fed chow plus skim milk (SM), chow plus SM fermented byS. thermophilus,chow plus SM fermented byL. bulgaricus,or chow plus water until sacrifice at 36 weeks, or before if moribund. Colon tumors were induced by s.c. injections of 1,2‐dimethylhydrazine hydrochloride during weeks 3 through 22. The control (chow + water) group received saline injections. The survival rate of the rats fed fermented milks was significantly higher than that of the rats fed nonfermented milk. The latter had a significantly higher incidence of ear‐duct tumors than the rats receiving fermented milk. The percentage of rats showing colon tumors was similar among all three experimental groups. The control group did not have any tumors. The rats receiving fermented milk had a significantly higher incidence of small‐intestine tumors than those receiving nonfermented milk. The rats onS. thermophilusmilk had the lowest percentage of malignant colon tumors of the three experimental groups. Results indicated that the feeding of fermented milks altered the metabolism of 1,2‐dimethylhydrazine and shifted the target organ from the ear duct to the small intestine. In addition, the colon tumor distribution for the fermented‐milk groups appeared to shift toward the anus.