The ability to infect human peripheral blood leukocyte-reconstituted severe combined immunodeficient (hu-PBL-SCID) mice with HIV has allowed evaluation of several strategies for preventing or treating infection. In one study, hu-PBL-SCID mice derived from HIV gp160-vaccinated donors were shown to resist HIV infection, and resistance correlated best with in vitro assays of cellular immunity. We have assessed directly the importance of cellular immunity to HIV in the present experiments by the adoptive transfer of HLA-A3-restricted HIV-1 Nef-specific or HLA-B14-restricted Gag-specific CD8+ CTL clones to SCID mice bearing HLA-matched or mismatched PBL grafts. Multiple inoculations of CTL before and after HIV-1 exposure protected HLA-matched hu-PBL-SCID mice from infection, but initiation of CTL therapy on the same day as HIV infection was much less effective. However, at the high numbers of CTL required for complete protection from HIV infection, many HLA-mismatched hu-PBL-SCID mice were also protected by pre-exposure CTL transfer. Transfer of CTL with a different specificity (HTLV-1 Tax) to HLA-matched hu-PBL-SCID mice also afforded partial protection. These results suggest that HLA-restricted cytotoxicity may be less important than other nonspecific effector mechanisms for the inhibition of HIV-1 infection in vivo.