Venous thrombosis is a risk factor for oral estrogen treatment in men and women. Platelet responses mediated through estrogen receptors may be involved in these processes. However, expression of estrogen receptors and estrogen receptor-associated protein have not been defined clearly in human platelets. Therefore, the present study was designed to characterize estrogen receptors and estrogen receptor-associated proteins in human platelets. Platelets were obtained from normal female and male blood donors between the ages of 25 and 65 years. Estrogen receptors (ERalpha and ERbeta), heat shock proteins (hsp70 and hsp90), endothelial nitric oxide synthases (eNOS), caveolin-1, -2 and -3 and calmodulin were analyzed in total platelet lysate by immunoblotting. Expression of ERalpha and ERbeta, hsp70, hsp90, eNOS, calmodulin, and caveolin-1 were observed in both sexes. Expression of all proteins was similar in platelets from males and females. Results of the present study suggest for the first time that both estrogen receptors alpha and beta, caveolin-1 and other proteins associated with estrogen receptors and production of nitric oxide are present in platelets of both males and females.