Graft-versus-host disease (GVHD) can occur in bone marrow-transplant recipients even when donor and host are identically matched at the major histocompatibility complex. GVHD in this context presumably arises because of differences in minor histocompatibility antigens. Murine GVHD to minor histocompatibility antigens has been studied in an effort to determine whether skin is a target of the immune response in this model system. T cell-depleted marrow cells (107) from B10.BR (H-2k) mice were supplemented with varying numbers of nylon wool-enriched splenic B10.BRT cells and transplanted intravenously into irradiated (1100 R) CBA (H-2k) mice. Sequential biopsies of ear skin were obtained at weekly intervals over a 7-week period. Histopathologic evaluation revealed basal cell layer vacuolization, exocytosis, and satellitosis of mononuclear cells in the epidermis. Dyskeratosis was observed only in animals receiving T cells, and proved to be the most reliable histologic parameter of disease with the number of dyskeratotic cells per linear millimeter of epidermis correlating both with severity of clinical disease and with the number of transplanted T cells. Ultrastructural examination revealed exocytosis of mononuclear cells into the epidermis where they were frequently apposed to degenerating and necrotic keratinocytes. These data indicate that the skin is an informative target organ for study of experimental GVHD to minor histocompatibility antigens. © 1986.