It has been well known that vanadium shows various physiological and pharmacological properties such as an insulin-mimetic effect. In view of the reported toxic effects there is the problems that the safety margin is narrow because of its strong toxicity, Vanadate was tested for its ability to cause blood aggregation. Although vanadate or $H_2O$$_2$ alone had little effect on platelet aggregation, treatment of vanadate and $H_2O$$_2$ together induced platelet aggregation indicated that it was occurred by pervandate or hydroxyl radical produced from the reaction of vanadate and $H_2O$$_2$. It was dependent on extracellular $Ca^{2+}$ion. Platelet aggregation caused by vanadate and $H_2O$$_2$ was inhibited by ascorbic acid, tocopherol, catalase, mannitol, and Tiron. In contrast to vanadate, vanadium yeast prepared by uptaking vanadate in yeast cells did not induce platelet aggregation in the presence of $H_2O$$_2$.>.