CD8 ÷ lymphocytes of the memory subset (= CD45R0 ÷) encompass antigen-specific effector cells, which are believed to be decisive for virus elimination in several viral infections. To determine whether this can be extended to HBV infection naive and memory T cells were studied among CD4 ÷-and CD8÷-lymphocytes and used monoclonal antibodies in two-color flow cytometric analysis to quantitate functional T cell subsets in peripheral blood of patients with acute hepatitis B (n = 11), chronic hepatitis B (n = 24) and healthy individuals (n = 26). Compared to CD4 ÷ populations of healthy individuals the number of total CD4 ÷ lymphocytes in patients with both acute or chronic hepatitis was significantly reduced. In contrast CD8 ÷ cells did not significantly change in either acute and chronic hepatitis. Analysis of naive and memory subsets demonstrated, however, a significant rise in CD45R0 ÷ memory cells from 5 to 15% (70% of all CD8÷cells) in acute hepatitis. These changes within the CD8 ÷ population were, however, restricted to the acute phase of hepatitis in that the frequency of CD8÷CD45R0 ÷ decreased within weeks post infection. Furthermore, patients with chronic hepatitis did exhibit normal values of CD8÷memory cells (30% of all CD8÷cells). These findings suggest that enrichment of CD8÷CD45R0 ÷ memory cells reflects an accumulation of functional effector ceils, which may be specifically activated by viral antigens and determine the outcome of infection.