Whole blood flow cytometry, an important new method for the assessment of platelet function, is particularly advantageous for neonatal studies because only minuscule volumes (approximately 2 microL) of blood are required. By this method, we have demonstrated that neonatal platelets are less reactive than adult platelets to physiological agonists in whole blood, as determined by the activation-induced increase in the platelet surface expression of P-selectin and the glycoprotein (GP) IIb-IIIa complex and by the activation-induced decrease in the platelet surface expression of the GPIb-IX complex. Our data suggest that the mechanism of neonatal platelet hyporeactivity is, at least in part, a relative defect in a common signal transduction pathway. We have further demonstrated that the platelets of very low birth weight (VLBW) preterm neonates are maximally hyporeactive on days 3 to 4 of life but return to almost the adult range by days 10 to 14. Given that intraventricular hemorrhage (IVH) is also maximal on days 3 to 4, these defects may contribute to the propensity of VLBW preterm neonates to IVH.