It has recently been reported that αB-crystallin, a low-molecular-weight heat shock protein, may be released from cells by mechanical stretch. We investigated a physiological role of αB-crystallin in platelet function. αB-crystallin inhibited platelet aggregation induced by thrombin or botrocetin in hamsters and humans. These platelets had specific binding sites for αB-crystallin. Moreover, αB-crystallin significantly reduced thrombin-induced Ca2 influx and phosphoinositide hydrolysis by phospholipase C in human platelets. Additionally, plasma levels of αB-crystallin were markedly elevated in cardiomyopathic hamsters. Levels of αB-crystallin in vessel walls after endothelial injury were markedly reduced. Therefore, our results suggest that αB-crystallin, which is discharged from vessel walls in response to endothelial injury, acts intercellularly as a regulator of platelet function.