A graft-versus-host reaction (GVHR) directed against DBA/2 non-H-2 antigens alone can be induced by grafting B10.D2 bone marrow and spleen cells intravenously to heavily irradiated, H-2d compatible (DBA/2 X B10.D2)F1 adult mice. Under the experimental conditions used, only 0-10% of recipients survive, but the survival is greatly increased by donor alloimmunization, a few days prior to grafting, against host-specific (DBA/2) non-H-2 antigens and non-specific (foreign) H-2 antigens. The increased survival is mediated by alloimmunization-activated suppressor cells which can decrease the intensity of the immune reaction developed by normal B10.D2 cells both in vivo (GVHR) and in vitro (proliferative response measured in mixed lymphocyte culture, MLC). The present experiments were designed to explore the antigenic requirements for inducing suppression. The results showed that in GVHR the protective effect induced by donor alloimmunization against the specific non-H-2 antigens, which leads to 70-80% survival, is due primarily, if not entirely, to immunization against Mlsa antigens. Results of MLC experiments confirmed this conclusion, showing that immunization against Mlsa antigens is sufficient to account for the suppressive effect induced by the specific immunization. In addition, they indicated that the non-specific protective effect induced by donor alloimmunization against foreign H-2 antigens, which leads to 20-30% survival, is due to immunization against antigens encoded by the K and/or I region(s) of the H-2 complex; immunization against D region encoded antigens alone has no effect.