The present study was undertaken to elucidate the mechanisms responsible for the cytotoxicity of herpes simplex virus (HSV)-specific CD4+ human cytotoxic T lymphocyte (CTL) clones, focusing on perforin and membrane-bound lymphotoxin (LT) (tumor necrosis factor-beta). Two HSV-specific CD4+ CTL clones, which expressed both perforin and membrane-bound LT, exerted HSV-specific cytotoxicity and cytotoxicity against LT-sensitive L929 cells. These CD4+ CTL clones lysed HSV-infected cells directly in an HLA class II-restricted manner and did not exhibit "bystander killing." The culture supernatants of these clones stimulated with HSV antigen showed no cytotoxicity against HSV-infected cells or L929 cells, suggesting that adhesion to target cells is essential to their antigen-specific and antigen-nonspecific cytotoxicities. The cytotoxicities of these clones against HSV-infected autologous cells were inhibited by an anti-CD3 monoclonal antibody but not by an anti-LT antibody. Conversely, their cytotoxicities against L929 cells appeared to be partially inhibited by the anti-LT antibody but not by the anti-CD3 monoclonal antibody. Furthermore, target cell DNA fragmentation induced by these CD4+ CTL clones was apparently observed in L929 cells but only faintly detected in HSV-infected autologous cells. L929 cell DNA fragmentation was also inhibited by adding the anti-LT antibody to CD4+ CTL cultures. These data suggest that some CD4+ CTL possess at least two cytolytic mediators, i.e., perforin and membrane-bound LT simultaneously, and can exert both antigen-specific cytotoxicity via two distinct mechanisms, necrosis and apoptosis.