Interleukin-12 (IL-12) plays a central role in mounting an effective cellular immune response directed towards elimination of intracellular pathogens. The present open-label, multicenter, dose-escalation phase I/II study was designed to assess tolerability, pharmacokinetics, pharmacodynamics, and efficacy of subcutaneously administered recombinant human interleukin-12 (rHuIL-12) in the treatment of chronic hepatitis C. Sixty patients (42 men, 18 women, aged 24-60) were treated with 0.03 μg/kg (n = 16), 0.1 μg/kg (n = 14), 0.25 μg/kg (n = 15), or 0.5 μg/kg rHuIL-12 (n = 15) for 10 consecutive weeks. rHuIL-12 was generally well tolerated, with 2 patients (3.3%) being withdrawn from treatment for adverse events. Treatment was associated with temporary decreases in neutrophils and lymphocyte counts and with elevations in serum transaminases and bilirubin. Serum IL-12 levels observed were higher at 0.5 μg/kg compared with 0.25 μg/kg doses, suggesting a dose-related increase in systemic exposure of IL-12. Measurable levels of interferon gamma (IFN-γ) were also observed at the highest dose of 0.5 μg/kg. At the end of treatment hepatitis C virus (HCV) RNA was detectable in all patients. A more than 50% decrease in pretreatment HCV RNA levels was observed in 3 of 16 patients of the 0.03-μg/kg dose group, in 3 of 14 of the 0.10-μg/kg dose group, in 6 of 15 of the 0.25-μg/kg dose group, and in 8 of 15 patients of the 0.5-μg/kg dose group. Although in several cases serum alanine transaminase (ALT) levels decreased either during or after treatment, ALT normalization was observed in only 4 patients at the end of treatment and in 5 patients at the end of follow-up. Significant anti–rHuIL-12 antibody titers were not detectable in any patient. In conclusion, antiviral activity of rHuIL-12 in patients with chronic hepatitis C does not appear advantageous in comparison with other currently available treatments.