The effect of highly purified human interleukin-2 (IL-2) was assessed on acetoxymethyl-methylnitrosamine(AMMN)-induced colorectal Sprague-Dawley rat adenocarcinoma. Treatment was given for 5 weeks and started 5 weeks after a period of 10 weeks intrarectal administration of 2 mg/kg of AMMN once a week. Animals which showed no evidence of tumors by endoscopical examination of the gut were given 10,000 units/kg of IL-2 s.c. into the dorsal region 5 days a week. Blood samples were analyzed from half of the animals before and 72 h after the end of the treatment. The median tumor volume and the median tumor number per rat were lower in the treated group than in the control. A significant decrease in response to two mitogens (PHA, Con-A) was seen in tumor-bearing rats concomitantly with the tumor growth. The immunological suppression was not restored by treatment with IL-2. No differences in natural killer activity or interferon levels were detected between the control and the treated group. These data indicate some antitumor effect of IL-2 given before the manifestation of tumors in AMMN-induced colorectal rat adenocarcinoma.