We examined the tumor-promoting activity of sodium taurocholate in the remnant stomach of rats. Ninety male Wistar rats, 8 weeks of age, were separated into four groups. In group I, the rats were given N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) at a concentration of 83 mg/l in drinking water for 15 weeks, and distal partial gastrectomy was performed by Roux-en-Y procedure to prevent duodenal reflux into the remnant stomach. Thereafter, a diet containing 0.25% sodium taurocholate was given for 43 weeks. The group II rats were given MNNG and gastrectomy and were then given the usual commercial diet. The rats in group III were given gastrectomy and sodium taurocholate and no previous administration of MNNG. Only MNNG was given to the rats in group IV. The incidence of malignant tumors in the remnant stomach was 40.9% (9/22), 27.3% (6/22), and 0% (0/22) in groups I, II, and III, respectively, while the incidence in the area corresponding to the remnant stomach (control) was 8.3% (2/24) in group IV. The difference in tumor incidence was statistically significant (P less than 0.05) between groups I and IV but not between groups II and IV, and not between groups I and II. Six of nine tumors in group I and all six tumors in group II were located in the anastomotic area. These results suggest that sodium taurocholate promotes tumor production in the remnant stomach, and that the surgical procedure may well be associated with this enhanced tumor occurrence.