An open label trial of GM-CSF plus high-dose interferon (IFN) alpha 2b was performed in eight patients with chronic hepatitis B infection and 16 patients with chronic hepatitis C, who either failed to clear virus with 6 months of daily high-dose IFN (5 MU daily) therapy (n = 22) or were considered untreatable because of advanced disease and leukopenia (n = 2). The dose of GM-CSF used was 500 micrograms subcutaneously twice weekly. The dose of IFN used was 5 MU daily. Both agents were administered for 4 months. Five of the eight hepatitis B patients and five of the 16 hepatitis C virus patients responded to combined therapy having previously failed IFN therapy alone. The hepatitis B virus responders had low entry ALT, AST, and gamma GPT levels as compared to the non-responders. No such differences for responders and non-responders were seen with the hepatitis C virus patients. These data suggest that the combination of GM-CSF and IFN may be more effective at achieving viral clearance than IFN alone.