Adhesion of leukocytes to vascular endothelial cells is a critical step in a variety of inflammatory conditions. We studied the expression and distribution of intercellular adhesion molecule-1 (ICAM-1) and endothelial leukocyte adhesion molecule-1 (ELAM-1) in frozen sections of 83 endomyocardial biopsy specimens from human allograft hearts using monoclonal antibodies and an avidin-biotin complex-alkaline phosphatase staining technique. Cases with cellular or humoral rejection and Quilty lesions were studied. Staining was graded from 0 to 3+ in lymphocytes and in capillary, arterial, venular, and endocardial endothelial cells. Expression of ICAM-1 in capillaries increased with the severity of cellular rejection and was prominent in humoral rejection. ICAM-1 was also expressed in lymphocytes in proportion to the degree of rejection. Little or no ELAM-1 expression was noted. In Quilty lesions the intensity of ICAM-1 expression was similar to that of mild-to-moderate rejection. Thus adhesion molecule expression can be identified in endomyocardial biopsy specimens of patients with rejection, suggesting a role for adhesion molecules in the process of rejection. These findings may prove useful in monitoring rejection and its response to therapy and in developing specific antisera directed against these molecules.