Platelet count and thrombopoietic activity were investigated in preterm infants at birth and during their first four months of life. Thrombopoiesis stimulating factor activity in cord serum was significantly lower than that of adults and of the respective mothers. No difference was noted between thrombopoietic activity in cord serum in the various gestational ages studiedthat is, 24 through 39 weeks. Preterm infants followed during their first four months of life showed a mean platelet count significantly higher than that observed in term infants at the respective age. Furthermore, thrombocytosis was found in premature thriving infants at the age of 1 and 2 months. It is suggested that this thrombocytosis is responsible for the low thrombopoietic activity observed in these infants during their first four months of life.