An extended phase II trial of human leukocyte (alpha) interferon was done in 48 patients with measurable metastatic renal cell carcinoma, 43 of whom were evaluable. Of these patients 1 (2.5 per cent) had a complete response that was maintained after 19 months, 6 (14 per cent) had a partial response and an additional 23 per cent had either a minimal response or stabilization of previously growing metastases for greater than 3 months. Toxicity caused termination of treatment in only 1 patient and generally was tolerable. Major toxicity consisted of fever (80 per cent), fatigue (80 per cent) and hematologic toxicity (42 per cent), which was severe in only 2 patients. Characteristics of patients responding to therapy were good performance status, previous nephrectomy and metastases limited to the lungs. The results achieved with human leukocyte interferon were superior to those achieved by immunotherapy or chemotherapy at this and other institutions, and further trials are warranted.