We have developed a murine model of acute and chronic forms of graft-versus-host disease (GVHD) as defined by clinical features that develop in response to minor histocompatibility antigen (minor HA) differences. C57BL/6J (B6) and LP/J mice were selected for serologic identity at H-2 and for mutual nonreactivity in mixed lymphocyte culture (MLC). Lethally irradiated B6 recipients were transplanted with anti-Thy-1.2− treated LP bone marrow cells plus various numbers of untreated LP spleen cells. The B6 recipient mice developed acute and chronic forms of GVHD that showed clinical and histological similarities to the acute and chronic forms of GVHD seen in human recipients of bone marrow transplants from HLA-identical and MLC-non-reactive donors. The definition of acute and chronic GVHD by clinical criteria appears to have biological significance in predicting subsequent survival patterns of recipient mice with GVHD. The incidence of acute and chronic GVHD in the mouse model was a function of the number of donor spleen cells transplanted. Using this model, we demonstrate that both acute and chronic forms of GVHD are initiated by donor lymphocytes. Based on these results, acute and chronic forms of GVHD induced to minor HA appear to be two manifestations of the same T-cell-mediated disease process rather than two different diseases.