The antiproliferative activities of recombinant rat gamma-interferon and recombinant human tumor necrosis factor alpha were evaluated in a new rat bladder tumor model, RBT323. The RBT323 is a spontaneously arisen, serially transplantable, bladder tumor in ACI rats. Histologically and ultrastructurally this tumor is a grade 2-3 transitional tumor without squamous cells. The RBT323 appears to be non-immunogenic in tumor excision and rechallenge experiments. The tumor was transplanted subcutaneously and the drugs were administered peritumorally. Gamma-interferon and tumor necrosis factor monotherapy starting two days after tumor implantation showed dose-dependent growth inhibiting effects in the dose range tested. Combination treatment with gamma-interferon and the highest dose of tumor necrosis factor resulted in synergistic antiproliferative effects. After cessation of the combination treatment all tumors resumed growth, initially with a low doubling time, but finally with their original growth rate. Gamma-interferon treatment initiated at a tumor volume of 0.2 to 0.5 cm.3 had no antitumor effects, whereas treatment with 100 micrograms. tumor necrosis factor per rat inhibited tumor growth significantly. Different combinations of the cytokines had synergistic effects against established tumors. These studies demonstrate antitumor effects of gamma-interferon and tumor necrosis factor in a rat bladder tumor model system. Combination therapy has synergistic effects and prolongs the time to tumor recurrence.