Blood from full-term newborns was compared to adult blood for its ability to regenerate prostaglandin I2(PGI2)-like activity from vascular tissue. The neonate possesses a markedly decreased ability to regenerate PGI2(0.10 ± 0.07 ng/mg vascular tissue) when compared to the adult (0.42 ± 0.12). This decreased activity was not due to the presence of an inhibitor in neonatal blood. The impaired ability of neonatal blood to regenerate PGI2-like activity was related to its markedly decreased antioxidant potential and was corrected (0.34 ± 0.08 ng/mg vascular tissue) by the addition of Vitamin Ein vitro.Plasma PGI2-like regenerating activity had normalized by 3 to 5 months of age (0.41 ± 0.11 ng/mg).SpeculationThe neonate demonstrates a normal bleeding time despite concomitant impairment in platelet function. Our finding of a deficiency of plasma prostaglandin I2(PGI2)-like regenerating activity may provide an explanation for the paradoxical normal neonatal bleeding time. Plasma PGI2-like regenerating activity normalized by 3 to 5 months of age, at a time when platelet function also is no longer impaired. The physiologic impairment in platelet function observed in the neonate is thus a teleologic necessity, providing a compensatory safety mechanism to counteract the prothrombotic tendency induced by a deficiency of plasma PGI2-like regenerating activity.