There is as yet no effective treatment for renal cell carcinoma and metastasizing malignant melanoma. This fact, coupled with in vitro investigations showing growth inhibitory or cytotoxic effects of interferon (IFN) on renal cell carcinoma and melanoma cell lines, led to phase II clinical trials with recombinant (r) IFN-alpha 2C and rIFN-gamma. So far 8 patients with renal cell carcinoma have been treated with IFN-alpha 2C and 3 patients with rIFN-gamma. There has been one complete response to IFN-alpha 2C, two mixed responses and one partial response. One patient on rIFN-gamma has stable disease and the other 2 have progressed. Eleven patients with metastasizing malignant melanoma were treated with IFN-alpha 2C. One patient achieved a complete remission and 3 others had stable disease which later progressed in 2 of them. Side-effects were reversible.