As a means of investigating further the pathogenesis of intestinal immunopathology, we have attempted to produce a destructive enteropathy by inducing an acute graft-versus-host reaction (GVHR) in mature, immunocompetent mice. Adult (C57b1/10 X DBA/2)F1 (BDF1) mice given C57B1/10(B10) spleen cells develop a severe GVHR which is associated with marked weight loss and high mortality. In the intestine an initial phase of enteropathy characterized by intense crypt hyperplasia is replaced by more severe intestinal damage which includes villus atrophy and loss of intra-epithelial lymphocytes. These pathological alterations are paralleled by the generation of anti-host cytotoxic T lymphocytes (CTL), marked immunosuppression and the loss of natural killer (NK) cells. In contrast to these findings, adult BDF1 mice given DBA/2 donor cells do not develop an acute systemic GVHR and have no CTL or intestinal pathology, despite prolonged splenomegaly and enhanced NK cell activity. Thus, destructive enteropathy can be induced during a GVHR in intact hosts and our results confirm that this enteropathy has a biphasic pattern, with villus atrophy representing the progression of initial crypt hyperplasia in severe forms of disease associated with weight loss and specific CTL.