Red blood cells (RBC) from patients with diabetes mellitus exhibit an increased propensity to adhere to cultured human umbilical vein endothelial cells (HUVEC) as a result of interaction of advanced glycation end products with their counter receptors, contributing to the pathogenesis of vascular complications. We determined whether the interaction of diabetic RBC with HUVEC induced cellular oxidant stress that would culminate in adherence and diapedesis of monocytes, these being initiating events in endothelial injury and atherogenesis. We show that the adherence of diabetic RBC (2% hematocrit), but not normal RBC, to HUVEC results in a fourfold increase in the production of lipid peroxides. Furthermore, diabetic RBC-induced oxidant stress causes a sixfold increase in platelet endothelial cell adhesion molecule-1 (PECAM-1) phosphorylation and doubles transendothelial migration of monocyte-like HL-60 cells; both are blocked by antioxidants and protein kinase C (PKC) inhibitors. Our results show that the adherence of diabetic RBC to endothelial cells initiates a cascade of cellular events resulting in PKC activation, causing PECAM-1 phosphorylation and concomitant transendothelial migration of monocytes. The increased diapedesis of monocytes, brought about by the interaction of diabetic RBC across vascular endothelium, may play an important role in accelerated atherosclerosis and cardiovascular disease in diabetics.