The immunological and haematological effects of continuous infusion of recombinant human interleukin-2 (rhIL-2) in 6 patients with metastatic melanoma and 6 with disseminated renal cell carcinoma are reported. In patients with malignant melanoma dacarbazine was given before IL-2; in renal cell carcinoma IL-2 alone was given. In malignant melanoma, 1 complete (CR) and 1 partial response (PR) were seen; 2 patients had stable disease (SD) and 2 progressive disease (PD). In renal cell carcinoma 4 patients had SD and 2 PD. Toxicity of IL-2 therapy was minimal. All patients showed increased cytotoxicity, that was not major histocompatibility complex restricted, towards target cells sensitive and insensitive to natural killer cells. These activities varied between individual patients and were less marked in cases of renal cell carcinoma. Cellular proliferative responses increased in all patients, being consistently higher following the first course of therapy, as did HLA-DR, CD16 and CD25 activation marker expression. Hypersegmentation of neutrophils and eosinophilia were commonly observed, and in renal cell carcinoma these changes were accompanied by abnormal lymphocyte morphology.