Monocyte adherence to the endothelium, their penetration to the subendothelial space and excessive lipid accumulation (foam cell formation) are the initial events in atherogenesis. Scavenger receptors have been reported to play an important role in foam cell formation, since modified low density lipoproteins can be taken up via scavenger receptors in a non-down-regulated fashion. In this study we demonstrate that stimulation of scavenger receptors in endothelial cells induces the expression of endothelial adhesion molecules. Polyinosinic acid (poly I), a known scavenger receptor ligand, significantly induced the expression of intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1) and E-selectin on human umbilical vein endothelial cells when compared with polycytidylic acid (poly C), a structurally related compound to poly I, which does not bind to the scavenger receptor. The effect of scavenger receptor ligands on the endothelial cell line EA hy. 926 was also tested. Poly I up-regulated ICAM-1 expression also on EA hy. 926 cells, while it had no effect on IL-1 beta or tumour necrosis factor-alpha (TNF-alpha) production on the same cell line. Poly I-induced ICAM-1 expression on EA hy. 926 cells could be inhibited by H7, a protein kinase C inhibitor, while HA 1004, a preferential protein kinase A inhibitor, had no effect on ICAM-1 expression. The role of protein kinase C in scavenger receptor-mediated adhesion molecule upregulation was confirmed by the ability of poly I to directly activate protein kinase C, when measured with 3H-phorbol dibutyrate binding to EA hy. 926 cells, while poly C again was ineffective.