Recombinant interferon alpha-C is a new strain of the alpha interferon family. It was given to 33 patients with measurable metastatic renal cell carcinoma of whom 31 were evaluable. Protocol consisted of 3 million U/d for 2 weeks, then 3 million U/m2 every other day until progression. No complete response was observed. Three patients (9.7%) had partial response for a mean duration of 5.6 months and eight patients (25.8%) were stabilized for a mean of 4.3 months. Responsive sites were mainly lung, bone, and kidney, while side effects were generally mild. better results were observed in previously nephrectomized patients who had not received chemotherapy or hormonotherapy for recurrent or metastatic disease (p less than 0.05), and also in patients with a brief disease-free interval and short delay from presenting symptoms of the primary tumor until interferon treatment (p less than 0.05). Median survival was significantly longer in responders than in progressors (p less than 0.05). We suggest that the efficacy of recombinant interferon alpha-C in a low-dose regime versus other types of interferon as first-line therapy for inoperable, metastatic, or locally recurrent renal cell carcinoma should be investigated in a prospective, controlled, randomized study.