In this study we have Investigated the role of CD4+, MHC class II-restricted cytotoxic T lymphocytes (CTLs) In the disease caused by lymphocytic choriomeningltls virus (LCMV) in β2-microglobulln deficient (β;2m-) mice. Intracranial (i.c.) infection with LCMV resulted in death of six out of 11 β2m- mice. Mice that survived showed a marked loss in body weight. Death and loss of body weight could be prevented by immunosuppressing the mice with irradiation or cyclosporine prior to i.c. injection of LCMV. This treatment also prevented induction of virus-specific, MHC class II-restricted CTL following peripheral inoculation with LCMV. In vivo depletion of CD4+ cells with antibody also prevented death following i.c. injection whereas in vivo depletion of CD8+ cells had no effect. Disease could be transferred to recipient β2m- mice by adoptive transfer of β2m- derived immune spleen cells. Transfer of non-immune spleen cells did not result in illness. In vitro treatment of immune spleen cells with anti-CD4 antibody and complement ellminated class II-restricted CTL activity and also prevented mortality of recipients after adoptive transfer. Treatment with anti-CD8 antibody had no effect. We were unable to transfer LCM disease to (β2m- recipients by adoptive transfer of immune spleen cells from C57BL/6 mice. These results suggest that, unlike normal mice, the pathology of LCM disease in β2m- mice is dependent upon virus-specific, CD4+CD8-, MHC class II-restricted T cells. © 1993 Oxford University Press.