Peripheral blood mononuclear cells (PBMC) from nonimmune healthy donors who did not have antibody to hepatitis A virus lysed hepatitis A virus-infected BS-C-1 cells to a greater degree than uninfected BS-C-1 cells. The predominant effector cells were contained in the nonadherent peripheral blood lymphocyte (PBL) fraction, although some lytic activity was associated with adherent cells. Characterization of the PBL with monoclonal antibodies showed that the responsible effector lymphocytes were contained in Leu-11+ and M1+ subsets, but not in the T3+ or T4+ subsets. The phenotypes of the effector cells active in the lysis of hepatitis A virus-infected cells are similar to those of human natural killer cells that lyse K562 cells. Human PBL produced high titers of interferon-alpha (IFN-alpha) when exposed to hepatitis A virus-infected cells. These results imply that hepatitis A virus infection may be controlled by lymphocyte responses in the liver, i.e., by lymphocyte-mediated lysis of the hepatitis A virus-infected cells, and by the production of high titers of IFN-alpha by lymphocytes exposed to hepatitis A virus-infected cells. Furthermore, these results, along with the observations that hepatitis A virus infection results in a persistent noncytocidal infection in vitro, support the hypothesis that lysis of hepatocytes infected with hepatitis A virus is by lymphocyte-mediated cytotoxicity and not by virus-induced destruction of the liver cell.