Antigen specific T-cell responeses are important for the eradication of Hepatitis B virus (HBV) in acute infection. Flow cytometric analysis of intracellular cytokines is a simple method for studying the T-cell response at the single-cell level in a short period pf time. In the present syudy, we invesfisated the hepatitis B virus (HBV)-specific T-cell response using flow cytometry. Eight patients with acute hepatitis B and 2 patients with fulminant hepatitis B were studied. Heparinized peripheral blood was incubated with recombinant HB core antigen (HBcAg), and intracytoplasmic cytokines were analyzed by flow cytometry. HBcAg-specific interferon-gamma positive CD4 T-cella were positive in 4 of 10 patients while HBcAg-specific interferon-gamma positive CD8 T-cells and interleukin-4 positive T-cells were negatiive in all patients. No intracytoplasmic cytokines were demonstrated in healthy subjects or with recombinant hepatitis C virus (HCV) antigens. HBcAg-specific CD4 T-cells could be detected by a sensitive flow cytometric analysis of the peripherad blood and the cytokine profile of these cells was Th1. These results suggest that HBcAg-specific Th1-type CD4 T-cells may play an important role in controlling viral clearance during acute HBV infection.