Interleukin-1 (IL-1) is an important mediator of inflammation and has been implicated in several forms of immunopathology. Here we have investigated whether IL-1 plays a role in the enteropathy which occurs during a graft-versus-host reaction (GVHR) in mice. Non-irradiated (CBA x BALB/c) F1 mice with GVHR had increased production of IL-1 and treatment with rabbit anti-IL-1 alpha antibodies abolished the crypt hyperplasia and significantly reduced the parallel increase in crypt length which occurs in the jejunum. Antibody treatment had no effect on the accompanying increase in intraepithelial lymphocyte (IEL) counts or on the splenomegaly. Recombinant IL-1 itself produced villus atrophy, crypt hyperplasia and increased IEL counts in normal mice and stimulated the proliferation of an intestinal epithelial cell line in vitro. We propose that IL-1 plays an effector role in immunologically mediated enteropathy, either via direct effects on epithelial cells or secondary to an action on other, stromal cells in the mucosa.