Thirty-three patients with renal cancer began treatment with human lymphoblastoid interferon (Wellferon) between August 1982 and February 1983. Interferon was administered as an i.m. injection at a dose of 5 X 10(6) units/sq m 3 times per week. Treatments were continued for at least 24 weeks in the absence of rapid disease progression or intolerable toxicity. Five patients demonstrated partial responses, which continued in two patients with durations of 239+ and 300+ days. Prolonged therapy was often required with a mean time to response of 99 days (22 to 190 days). Toxicity was substantial. Fever, chills, arthralgias, and myalgias occurred following most doses, but usually were well tolerated. Leukopenia and hepatic enzyme elevations were usually modest and always reversible. Dose-limiting side effects were progressive fatigue and anorexia which reversed within approximately 4 to 6 weeks after cessation of interferon therapy. There was no correlation between interferon levels, clinical toxicities, and response in this group of patients. We conclude that interferon has definite antitumor activity in renal cancer when given by this dose and schedule.