We have determined the minimal concentrations of 4 aggregating agents that were required to cause a 65–75 % increase in light transmission without disaggregation during the 5-min recording period in PRP samples of 14 neonates and 10 adults. We found, for example, that this degree of aggregation could be elicited in adult PRP by a mean value of 2.09 μM ADP, whereas similar aggregation of neonatal PRP required 5.16 μM ADP. Based on these figures, we observed that adult PRP was about 2.5 times more sensitive to ADP, at least 10 times to epinephrine and 2.3 times to collagen than neonatal PRP. However, neonatal PRP was approximately 20% more sensitive to ristocetin than adult PRP. By using concentrations of aggregating agents that caused comparable aggregation of neonatal and adult PRP, we noted comparable inhibition of aggregation in these samples by aspirin. While neonatal PRP was less sensitive than adult PRP to physiological aggregating agents, there was no evidence that the former was more susceptible to in vitro aspirin inhibit