The present studies were undertaken to examine the ability of a viral pathogen to enhance immune alterations associated with parent → F1graft-versus-host re actions (GvHR) across defined donor/recipient MHC genetic disparities. Murine cytomegalovirus (MCMV) was administered concurrently with a parental lymphoid inoculum into unirradiated F1recipients in strain combinations limiting allogeneic differences to the entire MHC complex (class I/II), the H-2K region (class I), or H-2IA (class II) regions only. Alterations previously found to be associated with GvHR involving changes in the expression of Ly-6, Lyt-2, and L3T4 were examined to characterize the effects of MCMV. Mice receiving low numbers of class I/II-disparate parental cells or MCMV alone failed to exhibit significant GvHR-associated changes. In contrast, introduction of cells and virus resulted in marked alterations characteristic of F1recipients injected with a large parental cell inoculum alone. Concurrent virus and parental cells could also induce marked changes when administered across differences involving only a class I—disparate—but not class II—disparate only—P → F1combination. In addition to the phenotypic changes observed during the con current virus and class I GvHR, markedly reduced spleen cell proliferative activity and associated weight loss and mortality appeared to indicate that virus had enhanced this reaction. In total, these findings demonstrated that a donor/recipient class I MHC difference was necessary for virus and parental cells to induce the changes observed, and thus not all donor/recipient antigenic differences will result in a similar virus-induced effect. The results are discussed with respect to the potential mechanisms that may account for the apparent exacerbation of GvHR-associated alterations.