The in vitro effect upon platelet aggregation of estradiol and synthetic estrogens (prolame, buame, and proacame) is described. Prolame and buame produced a dose-dependent inhibition on platelet aggregation. Estradiol and proacame did not show anti-aggregating effects. The results suggest that the use of prolame and buame in estrogen therapy could reduce the risk of thrombo-embolic accidents.