A phase II clinical trial of subcutaneous recombinant Interleukin 2 (rIL-2) given by 5 days pulses followed by a 9 days rest has been performed in patients affected by renal cell carcinoma, malignant melanoma and colorectal cancer. A total of 25 patients entered the study, completed at least six courses of treatment, and were evaluable for toxicity and response to treatment. This schedule of subcutaneous rIL-2 was well tolerated and no World Health Organization grade 3 side effects were observed. A 33.3% response rate was recorded in patients affected by renal cell carcinoma, although no major responses were achieved in patients with malignant melanoma and colorectal cancer. A durable increase of natural killer activity retained by poeripheral blood mononuclear cells was demonstrated in these patients and was paralleled by increased serum levels of interferon gamma and tumor necrosis factor a without changes of circulating interleukin-1d. It is concluded that this schedule of pulse administration of subcutaneous rIL-2 has antitumor activity in renal cell carcinoma and produces durable biomodulatory effects.