Twenty patients with measurable metastatic renal cell carcinoma (RCC) were treated with interferon alpha-2a (18 X 10(6) IU i.m. 3 times weekly) in combination with vinblastine sulfate (0.1 mg/kg i.v. every 3 weeks). Objective responses in the lungs, bone and liver metastases were observed in 5 of 18 evaluable patients. Dose reduction of interferon alpha-2a (to 9 X 10(6) IU i.m. 3 times weekly) was necessary in 7 patients due to intolerable flu-like side effects and leukopenia (nadir 3,500 leukocytes/mm3). Tolerance was good in 55% of cases. Objective clinical response was observed in 27.7% of patients, and only 38.8% progressed. It is necessary to perform further studies, varying the therapeutic schedules, in order to elicit a better control of the toxic effects and a greater objectiveness of the clinical response.