The mechanism(s) through which combination treatment with Adriamycin and recombinant interleukin 2 (rIL2) affects murine renal cell carcinoma was investigated. A single dose of Adriamycin (ADM) administered 1 day after implantation of tumor cells s.c. delayed the formation of tumor and significantly extended the median survival time. About 40% of the treated mice remained tumor free for more than 6 months. Treating mice with a similar dose of ADM 12 to 16 days after implantation of tumor caused complete regression of established tumors within 10 to 12 days in more than 90% of the mice. However, after a transient tumor free period of 15 to 20 days recurrence of tumor was observed in all treated mice. In contrast, a regimen that included treating tumor bearing mice with a single dose of ADM followed by a daily i.p. injection of rIL2 5 x 10(4) units for 10 days delayed the recurrence of tumors and significantly prolonged the survival time compared to the median survival time of mice treated with ADM alone. About 20 to 30% tumor bearing mice remained tumor free for 4 months following treatment with ADM and rIL2. Treatment with rIL2 alone produced no antitumor response. In addition, rIL2 itself was not inhibitory for tumor cell growth nor did it modulate the cytotoxic response of renal cell carcinoma cells to ADM in vitro. Mice that were cured following treatment with ADM and rIL2 were resistant to a rechallenge with viable tumor cells, and cured mice also expressed a tumor specific T-cell mediated delayed hypersensitivity reaction. The immune cells that mediate tumor rejection were identified as Thy-1.2+ T-cells. Taken together, these results indicate that antitumor activity of combined Adriamycin/rIL2 treatment is at least partly attributable to the production of tumor specific immunity.