Injection of parental spleen cells into unirradiated F1 hybrid mice results in suppression of the potential to generate cytotoxic T lymphocyte (CTL) responses in vitro. In an attempt to protect the F1 mice from immunosuppression, the recipients were injected with antibodies specific for major histocompatibility complex (MHC)-encoded antigens of the F1 mice 24 h before inoculation of the parental spleen cells. 8-14 d later, the generation of CTL responses in vitro against H-2 alloantigens was tested. Alloantiserum directed against either parental haplotype of the F1 strain markedly diminished the suppression of CTL activity. Furthermore, monoclonal antibodies recognizing H-2 or Ia antigens protected the F2 mice from parental spleen cell-induced suppression. Although this study has been limited to reagents that recognize host H-2 determinants, these findings do not necessarily imply that protection against graft vs. host (GvH) can be achieved only with anti-MHC antibodies. However, protection was observed only by antibodies reactive with F1 antigens, and small amounts of the alloantibodies were sufficient to diminish CTL suppression. Adoptive transfer of spleen cells from syngeneic F1 mice treated with anti-h-2a alloantiserum 24 h previously provided protection equal to that of injection of the recipients with alloantibodies. The cells necessary for this effect were shown to be T cells and to be radiosensitive to 2000 rad. This cell population is induced by antisera against F1 cell surface antigens and effectively counteracts GvH-associated immuno-suppression.