Cytotoxic T cells have been identified in the peripheral blood of patients with acute hepatitis B virus infections for a short period after clinical presentation. However, in patients in whom the virus persists these have been difficult to demonstrate. In the chronic infection during HBe antigen clearance, when there has been an exacerbation of the disease, we have been able to demonstrate MHC class I-restricted cytolytic response directed against the nucleocapsid antigens. In an HLA-A2 patient this was induced in vitro with the peptide pl8-27, previously described as an HLA-A2-restricted T cell epitope. In patients of other HLA types, recombinant core antigen was used to induce antigen-specific lysis: statistical analysis of the cytolytic responses of chronically infected patients demonstrated a nucleocapsid antigen-specific lysis in patients who were seroconverting. Removal of CD4+ cells reduced non-MHC-restricted cytolysis, allowing an MHC class I-restricted cytolytic component to be demonstrated.