The epidemiologic patterns of large bowel cancer have implicated a diet high in beef as a risk factor. Another study has shown less colon cancer in Finns than in Danes. Among the possible explanations were the consumption of milk and fermented dairy products and the numbers of lactobacilli in the fecal flora, both of which were higher in the Finns. As a test of these hypotheses, 20 mg 1,2-dimethylhydrazine dihydrochloride/kg body weight was administered for 20 weeks to male inbred F344 rats. Rats consuming a beef diet simulating a "Western" diet had higher rates of colon cancer than did grain-fed rats (83% vs. 31%, respectively). Viable cultures of Lactobacillus acidophilus were fed to rats eating the meat diet; the rats were then challenged with 1,2-dimethylhydrazine dihydrochloride. The colon cancer incidence after a 20-week induction period was lower in the animals receiving the L. acidophilus (40% vs. 77% in controls), but no difference in incidence was discerned after a 36-week period. Thus dietary supplements of L. acidophilus seemed to increase the latency or induction time for experimental colon cancer.