The aim of this study was to evaluate the safety profile of s.c. administered recombinant human interleukin 12 (rHuIL-12). Pharmacokinetics and pharmacodynamics of rHuIL-12 and any evidence of antitumor effect were also considered. Ten pretreated patients with progressive metastatic melanoma were enrolled in this pilot study. Patients received a fixed dose of rHuIL-12 (0.5 microgram/kg) for two identical 28-day cycles, with injections given on days 1, 8, and 15 of each cycle. In case of any evidence of response or disease stabilization, the treatment was continued for two further 28-day cycles. Toxicity mainly consisted of a flu-like syndrome. Transient increases in transaminasemia (6 of 10 patients) and triglyceridemia (8 of 10 patients) were observed. Peak serum IL-12 levels were reached 8-12 h after the first injection in all patients; no serum IL-12 was detectable in 6 of 9 evaluable patients after the last injection of the second cycle. No antibody response to rHuIL-12 could be detected in any of the patients. A marked, transient reduction in circulating CD8+ and CD16+ lymphocytes and neutrophils was observed after the first administration and high levels of serum IFN-gamma and IL-10 were detected in all patients within 24-48 h. Tumor shrinkage, not reaching partial or complete remission, involved the regression of s.c. nodules (2 of 3 patients), superficial adenopathies (1 of 3 patients), and hepatic metastases (1 of 3 patients); regressions were detected after the first cycle of treatment and were maintained in spite of progression at different sites. s.c. rHuIL-12 treatment was well tolerated and had marked effects on immune parameters and potential antitumor activity.