Recombinant interleukin-2 (rIL-2; EuroCetus, Amsterdam, Netherlands) was studied in an outpatient phase II trial in 14 patients with progressive metastatic renal carcinoma, malignant melanoma, and colorectal cancer. Escalating doses of rIL-2 were administered as subcutaneous bolus every 12 hours, starting at 0.3 million U/m2/d. A 100% dose increase occurred at weekly intervals, up to a maximum of 2.4 million U/m2/d. Responding patients or patients with stable disease after 4 weeks of rIL-2 (n = 9) were continued on maintenance therapy at 1.8 million U/m2 of rIL-2 administered once weekly. After 12 weeks of therapy, one renal cell cancer patient had a partial regression in lung metastases. Bolus injection of rIL-2 (1.2 million U/m2) resulted in peak serum levels of 25 to 30 U/ml. Toxicity of this regimen was moderate, with local inflammation at the injection sites, grade I-II (World Health Organization) malaise, nausea and/or vomiting, and fevers in 70% to 100% of patients treated. Thyroid dysfunction was observed in 10 patients receiving subcutaneous rIL-2; four of these patients had laboratory evidence of hyperthyroidism, and one had hypothyroidism. rIL-2-induced toxicity reversed spontaneously after cessation of treatment. In all patients receiving rIL-2, a dose-dependent increase in peripheral blood lymphocyte and eosinophil counts was noted, with a mean of 2.6 and 3.8 x 1,000/microliters after 4 weeks of therapy; mean lymphocyte and eosinophil counts were measured at 2.0 and 2.4 x 1,000/microliters in patients who received prior high-dose chemotherapy, compared with 3.2 and 5.1 x 1,000/microliters in those who did not.(ABSTRACT TRUNCATED AT 250 WORDS)