['Little information exists concerning platelet function in neonates due to the small blood volume. Most studies using conventional aggregation methods have shown a diminished response to various agonists. This is in contrast to the lack of a bleeding tendency and to a short bleeding time in healthy neonates. In previous work we have shown that in healthy term neonates even after an uncomplicated delivery signs of thrombin generation can be demonstrated. This activation of the clotting system may also lead to platelet activation in the neonate. We investigated by means of flow cytometry the expression of activation markers GMP 140 and GP 53 on the surface of neonatal platelets and GP 53 intracellularly by means of a newly developed assay. The expression of GMP 140 and of GP 53 after thrombin stimulation was significantly higher on adult rather than neonatal platelets, while there was no difference between neonatal and adult platelets using GP 53 as intracellular marker. In none of the healthy term neonates after an uncomplicated delivery were activated platelets demonstrated.'] Is hyporeactivity of neonatal platelets caused by preactivation during birth?