The mechanism of heparinized blood accumulation during cardiopulmonary bypass(CPB)surgery remains obscure, although a change in erythrocyte shape(echinocytes)and thrombus formation have been postulated as causes. We examined platelet aggregation in vitro and hemostatic effects under conditions of alkalemia to confirm the previously reported theory of a change in erythrocyte shape. Despite the formation of platelet aggregates comprising particles <25µm at alkaline load(pH 9.0), large platelet aggregates of particles >50µm did not form under alkaline conditions. In addition, activated partial thromboplastin time(APTT)and prothrombin time(PT)were significantly prolonged. Therefore, heparinized blood accumulation during CPB might be due to a change in erythrocyte shape(echinocytes)and not to thrombus formation.