Since mononuclear cells are recruited in atherosclerotic lesions, the expression of adhesion proteins by the arterial endothelium may play a major role in atherogenesis. The relationships between ICAM-1, E-selectin, and VCAM-1 expression on the arterial endothelium and the presence and degree of maturation of intimal macrophages in human atherosclerotic lesions was investigated. By quantitative double immunostaining with a pan-macrophage-specific monoclonal antibody, HAM-56, and a recently developed monoclonal antibody that is specific for mature macrophages, 3MA-B38, arterial sections were classified as (I) normal, (II) thickened without macrophage infiltration, (III) atherosclerotic with recent macrophage infiltration or (IV) atherosclerotic with infiltration of mature differentiated macrophages. A marked increase in the expression of ICAM-1, E-selectin, and VCAM-1 was observed on endothelial cells adjacent to recently recruited macrophages. Endothelial cells overlying differentiated macrophages exhibited a lower but significant increase in VCAM-1 expression, with no difference in ICAM-1 and E-selectin expression with respect to that observed in endothelium of normal arteries. These findings indicate that the endothelium covering the human arterial wall exhibits different states of activation as reflected by the expression of adhesion proteins, and that intimal monocyte/macrophage recruitment appears to depend on the level of expression of adhesion proteins.