The graft-versus-host reaction (GVHR) in both mice and humans can lead to the development of a broad spectrum of clinical and pathological symptoms. These symptoms are strikingly similar to those of a number of diseases of proven or presumed immunological origin, such as systemic lupus erythematosus (SLE), other collagen vascular diseases, lymphoproliferative disease, and aplastic anemia. The purpose of our investigation was to describe the immunological and pathological events that take place in the course of graft-versus-host disease (GVHD) and to gain insight into the cellular mechanisms underlying these events. To this end, a model was employed in which nonirradiated F1 mice were used as recipients of parental lymphoid cells. By pathological manifestations, 2 basic forms of GVHD can be distinguished in such non-irradiated F1 recipients: One is acute GVHD which is often lethal. It is characterized by a variety of suppressive (hypoplastic) pathological symptoms, including a severe hypoplasia of the lymphohemopoietic system accompanied by aplastic anemia and hypogammaglobulinemia. The other basic form is characterized by stimulatory symptoms, such as persistent lymphoid hyperplasia, formation of autoantibodies, and development of pathological symptoms reminiscent of SLE and other collagen vascular diseases. The suppressive pathological graft-versus-host (GVH) symptoms are caused by T suppressor/killer (TS/K) cells of the donor which react towards allogeneic class-I-structures of the F1 recipient's major histocompatibility complex (MHC). The stimulatory pathological GVH symptoms, by contrast, are caused by donor T helper (TH) cells which react toward the recipient's allogeneic class-II-MHC structures. The possible implications of these observations for the pathogenesis of a number of GVH-like diseases in humans are discussed. The hypothesis is advanced that some of these GVH-like conditions, which arise either e causa ignota or after exposure to certain viruses or drugs, are caused by T lymphocytes reacting against self-MHC structures on lymphohemopoietic cells that were rendered "foreign". By analogy to GVHD, it is conceivable that the development of either stimulatory or suppressive GVH-like symptoms in individuals exposed to a given virus or sensitizing drug depends not on the etiologic agent per se, but on whether the predominant response is made by the individual's TH or TS@K cells. This, in turn, might depend on whether the agent becomes immunogenic in combination with class-II or class-I alloantigens.