The effect of a local delayed hypersensitivity reaction (allograft rejection) on small intestinal architecture has been examined by using scanning electron microscopy of the mucosal surface. In isografts, regarded as a morphological basis for normality of the system, the villi appeared normal and finger like. In allografts the appearances ranged from fairly normal areas to others with stunted and ridged villi, and flat areas in which crypt mouths opened on to the flat mucosal surface. The fine structure of epithelial cells was studied further by transmission electron microscopy. There was an apparent reduction in the size of the microvillous border in allografts but no other consistent abnormality. The similarities between this model of allograft rejection, and untreated coeliac disease in man, are highlighted and contrasted with the effects of acute radiation injury on the intestine.