We studied cell-mediated cytotoxicity to hepatitis A virus-infected cells in seven patients with acute type A hepatitis and two controls. Skin fibroblast cultures obtained from the skin biopsies of seven patients after acute hepatitis A virus infection and from two persons without history of current or past hepatitis A virus infection were inoculated with hepatitis A virus. Infection of fibroblast cultures always resulted in an inapparent, persistent infection with production and release of infectious hepatitis A virus. Peripheral blood lymphocytes were collected from the same patients at different times after onset of icterus and were stored in liquid nitrogen. Cytolytic activity of peripheral blood lymphocytes was determined by a microcytotoxicity assay using autologous 51Cr-labeled hepatitis A virus-infected and uninfected target cells. Cytotoxic peripheral blood lymphocytes capable of lysing autologous hepatitis A virusinfected skin fibroblasts were detected in all patients with hepatitis A but were not demonstrable in the controls without antibodies against hepatitis A virus. The clinical course of the hepatitis A virus infection was normal in five patients; and in these patients, cytolytic activity of peripheral blood lymphocytes against hepatitis A virus-infected autologous targets peaked 2 to 3 weeks after onset of icterus. A clinically protracted form of the disease with persistent elevation of aminotransferase for at least 5 months after onset was present in two patients. In these cases, the highest cytolytic activity was demonstrated in peripheral blood lymphocytes collected 8 to 12 weeks after onset of icterus. Natural killer cell activity, as measured by specific lysis of K562 cells, did not show peak activity during the acute or convalescent phases of the disease. Our experimental findings suggest that cytolytic T cells play an important role in the pathogenesis of hepatitis A virus infection.