We have examined the interaction of freshly isolated human blood monocytes with cultured human umbilical vein endothelial cells in vitro. Purified monocytes incubated with confluent primary or passaged endothelial cells (EC) for 90 min at 37 degrees C bound at maximal densities of 6.5-7.0 X 10(3)/mm2 (8 or 9 per EC) without causing disruption of the monolayer. Monocyte-EC binding proceeded in the presence of plasma proteins or optimal phagocytic doses of opsonized zymosan particles. The avidity of attachment was not diminished by alternative monocyte isolation techniques. Monocyte attachment to EC was dependent upon the presence of divalent cations (magnesium greater than calcium) and was inhibited at 4 degrees C. Monocytes selectively bound to EC when incubated with monolayers composed of smooth muscle cells and EC. Neither EC monolayer confluence nor a variety of EC culture conditions affected the high levels of monocyte binding. In contrast, human neutrophils (less than 1 per EC) and lymphocytes (less than 2-3.5 per EC) bound at lower maximal densities under the same conditions, while platelet reactivity remained minimal. The distinctively higher affinity of human blood monocytes relative to other circulating white cells for binding to cultured human EC may have relevance to their function in vivo.