To extend our investigations on the interaction between diabetic platelets and endothelium, we tried to identify the molecular components involved in the increased adhesiveness of diabetic platelets to cultured valvular endothelial cells (VEC). Platelets from diabetic patients were radiolabeled with (3H]-adenine, incubated for 30 min at 37°C with confluent VEC grown in medium containing 4.5 g/L glucose, and the monolayer-associated radioactivity was used to calculate the adhesion index. To identify the plasmalemma proteins involved in the adhesion process, platelets were incubated for 30 min prior totheadhesion assay with one of the following monoclonal antibodies: AP-2 (anti GP IIb-IIIa), AP-5 (anti GP IIIa), TM 83 (recognizes an epitope other than the fibrinogen binding site in GP IIIa), PECAM 1.2 (anti PECAM-1) or a polyclonal anti-fibronectin receptor (anti FnR). In addition, two synthetic peptides, RGDS and GPRP, applied alone or together, were used. The effect of paraformaldehyde fixation of diabetic platelets on their adhesion was also tested. The results showed that except for TM 83, all antibodies reduced significantly (∼45%) the adhesion index of diabetic platelets to VEC. The synthetic peptides also decreased the adhesion by∼30%. Paraformaldehyde-fixed diabetic platelets fail to adhere to VEC. Taken together these observations suggest that: (1) platelet GP IIb-IIIa complex, PECAM-1 and FnR may be instrumental in the increased adhesion of diabetic platelets to VEC; (2) fibrinogen binding sites in the GP IIb-IIIa complex and fibrinogen/fibrin are important contributors to the adhesion process and (3) impairment of diabetic platelets adhesion by chemical fixation, supports the role of cytoskeletal proteins reorganization and redistribution of some plasmalemma components during adhesion.