The intercellular adhesion molecule-1 (ICAM-1) is a ligand for the beta2 integrin. It is present on the vascular endothelium and plays an important role in cell to cell interaction in lymphocyte recirculation. ICAM-1 is expressed not only on the endothelium but also on other cells, especially lymphocytes adhering to the endothelium, B cells and mitogen-stimulated T cells. To define the functional role of ICAM-1 in lymphocytes, the three-dimensional distribution of ICAM-1 on the surface of lymphocytes in the rat mesenteric lymph node HEV was investigated by immunoscanning electron microscopy. ICAM-1 expression was closely associated with variations in surface structures of lymphocytes, which showed a cap-like structure, membrane ruffles and slender microvilli. ICAM-1 was preferentially localized on the cell surface including membrane ruffles (about 90% of all counted gold particles), but rarely on the microvilli (10%). The results suggest that activated lymphocytes with variations in surface structures demonstrate moderate expression and preferential localization of ICAM-1. This seems to enable a three-dimensional interaction of the microvilli of one lymphocyte with the surface membrane of another cell and might be a prerequisite for cluster formation between them, as seen in antigen presentation.