Male Sprague-Dawley rats, 25 days of age, were placed on a control ration and diets containing trypsin (2429 u/g) and tamoxifen (initial level: 4 PPM) at which time, 1,2-dimethylhydrazine was injected s.c. at 20 mg base/kg and continued once/week for 20 weeks. Most of the animals were killed 65 days after injection 20. In view of weight losses, the tamoxifen supplement was decreased to a final level of 0.50 PPM without intervening control diet feeding. The total number of colon adenocarcinomas and the distribution in the proximal and distal portions did not differ significantly from the respective controls and the tumor frequencies in the small intestine were not remarkable. However, the general animal conditions, weight changes and the presence of other tumor types were more extreme as compared to a similar trypsin supplement reported for rats administered carcinogen by gavage once weekly for 15 consecutive weeks. With the latter series, colon adenocarcinoma frequencies were markedly decreased.