Immunocytochemical analysis of endomyocardial biopsies from cardiac transplant patients has defined changes in expression of antigens, expressed on both the myocardium and endothelium which are characteristic of rejection. Biopsies taken from normal donor heart (prior to transplantation) have been compared with biopsies showing histological signs of rejection. There is induction of MHC class I antigen on the normally negative myocardial plasma membrane and induction of the adhesion molecule ICAM-1 on the intercalating discs. Capillary endothelial cells, which constitutively express Class II DR antigen and ICAM-1 in normal heart show increased of endothelial antigens Pal-E and FVIII-RA during rejection. These results demonstrate perturbation of the endothelial system during rejection and possibly indicate damage to the capillary endothelial cells.