β2-Microglobulin-deficient (β2m-) mice generate a CD4+ major histocompatibility complex class II-restricted cytotoxic T-lymphocyte (CTL) response following infection with lymphocytic choriomeningitis (LCM) virus (LCMV). We have determined the cytotoxic mechanism used by these CD4+ CTLs and have examined the role of this cytotoxic activity in pathogenesis of LCM disease in β2m- mice. Lysis of LCMV-infected target cells by CTLs from β2m- mice is inhibited by addition of soluble Fas-Ig fusion proteins or by pretreatment of the CTLs with the protein synthesis inhibitor emetine. In addition, LCMV-infected cell lines that are resistant to anti-Fas-induced apoptosis are refractory to lysis by these virus-specific CD4+ CTLs. These data indicate that LCMV-specific CD4+ CTLs from β2m- mice use a Fas-dependent lytic mechanism. Intracranial (i.c.) infection of β2m- mice with LCMV results in loss of body weight. Fasdeficient β2m-.lpr mice develop a similar wasting disease following i.c. infection. This suggests that Fas-dependent cytotoxicity is not required for LCMV-induced weight loss. A potential mediator of this chronic wasting disease is tumor necrosis factor (TNF)-α, which is produced by LCMV-specific CD4+ CTLs. In contrast to LCMV-induced weight loss, lethal LCM disease in β2m- mice is dependent on Fas-mediated cytotoxicity. Transfer of immune splenocytes from LCMV-infected β2m- mice into irradiated infected β2m- mice results in death of recipient animals. In contrast, transfer of these splenocytes into irradiated infected β2m-.lpr mice does not cause death. Thus a role for CD4+ T-cell-mediated cytotoxicity in virus-induced immunopathology has now been demonstrated.