The effect of graft-versus-host reaction on the course of concommitant retrovirus-induced lymphoproliferative disease was investigated. The graft-versus-host reaction was elicited by a single i.v. injection of 1.2 x 10(8) parental spleen cells into adult F1 mice. Lymphoproliferative disease was induced by a single transfusion of 0.2 ml of whole blood from donors with fully developed disease, induced by infection with retrovirus LP-BM5 MuLV. Graft-versus-host reaction and the lymphoproliferative disease each separately produced similar syndrome consisting of splenomegaly, lymphadenopathy, leukopenia, neutrophilia, reduced in vitro proliferation of spleen cells and suppression of in vivo immune responsiveness. The above symptoms were usually less pronounced during graft-versus-host reaction. Ongoing graft-versus-host reaction neither aggravated nor accelerated the course of the virus-induced lymphoproliferative disease in genetically susceptible F1 hybrids. Likewise, an ongoing graft-versus-host reaction in genetically resistant F1 hybrids did not alter their susceptibility to the retrovirus infection. The apparent lack of the effect of graft-versus-host reaction -dependent immunosuppression on the severity and the course of the concommitant retrovirus-induced lymphoproliferative disease suggests pathogenic differences between the murine syndrome and human AIDS for which the murine disease is considered by some to be an animal model.