To examine the possible role of lymphocytes in the course of hepatitis B virus infection, we studied peripheral blood lymphocytes and Epstein-Barr virus transformed B-cells for their capability to produce hepatitis B virus gene products. Infection of these cells with a recombinant vaccinia virus containing the hepatitis B virus surface antigen resulted in the production and secretion of hepatitis B virus surface antigen shortly after infection reaching a peak after three days. When the supernatants of the cells were analyzed on density gradients, a peak of reactivity for hepatitis B virus surface antigen was reached at 1.21 g/cm3. Electron microscopy revealed the presence of spherical and filamentous HBsAg particles. These findings show that human lymphocytes are capable of producing hepatitis B virus surface antigen.