Albumin-interferon-alpha (IFN-alpha) is a novel 85.7-kDa recombinant protein consisting of IFN-alpha that is genetically fused to human serum albumin. In this Phase I/II, multicentre, open-label study, we evaluated the safety and tolerability, pharmacokinetics and pharmacodynamics of albumin-IFN-alpha in IFN-alpha-experienced patients with chronic hepatitis C. Albumin-IFN-alpha was administered in 22 escalating doses (7-900 microg) in a single injection or in two injections 14 days apart. In the 119 patients studied, there were no discontinuations because of adverse events, and albumin-IFN-alpha had a favourable safety profile at doses up to 900 microg. The most common adverse events were headache (56%), fatigue (52%), injection site erythema (38%), arthralgias (32%) and pyrexia (27%). Reduced clearance resulted in a mean elimination half-life of 159 h, which supports dosing at 2- to 4-week intervals. Induction of the IFN-specific gene OAS1 was maintained for > or = 28 days following a single injection of albumin-IFN-alpha at doses of > or = 40 microg. Dose-dependent antiviral activity was observed in this IFN-alpha-experienced study population. Antiviral activity of > or = 1.0-log reductions in HCV RNA was observed in 47% (37/78) of patients in the 120- to 900-microg cohorts and in 59% (16/27) in the 400- to 900-microg double-injection cohorts. These results support further clinical studies of albumin-IFN-alpha for the treatment of patients with chronic hepatitis C.