We have tested the hypothesis that interferon-gamma (IFN-gamma) plays a role in the enteropathy of graft-versus-host reaction (GVHR) by treating host mice with a monoclonal antibody directed at this mediator. Two models of GVHR were examined. In the mild proliferative GVHR, which occurs in adult unirradiated (CBA x BALB/c)F1 mice given parental spleen cells, anti-IFN-gamma slightly inhibited the development of splenomegaly and the activation of natural killer (NK) cells in GVHR. Anti-IFN-gamma had no effect on splenomegaly or generation of anti-host cytotoxic T lymphocytes (CTL) during the more severe GVHR in adult BDF hosts, but inhibited the weight loss and mortality normally found in this GVHR. Despite these variable effects on systemic GVHR, anti-IFN-gamma treatment abolished the crypt hyperplasia and increased counts of intraepithelial lymphocytes (IEL) normally found in the jejunum of (CBA X BALB/c)F1 mice with GVHR. In parallel, anti-IFN-gamma-treated BDF1 mice with GVHR did not develop the villus atrophy and intense crypt hyperplasia found in untreated GVHR hosts. These results support the view that IFN-gamma is essential for the development of enteropathy in GVHR and we propose that this mediator may also be involved in the pathogenesis of clinical enteropathies in man.