The aggregation of blood platelets and the retraction of the clot were measured during the first 21 days of life in the blood of premature and newborn infants. The results were compared with those of children at the age of 6 to 14 years. At first the aggregation of blood platelets is diminished in premature and newborn infants in comparison with the results of school children. But in the period of 3 weeks of investigation a rise in the platelet aggregation is seen.The clot retraction of newborn babies in the first days of life is the same as in healthy school children. After this time the weight of the clot rises in premature and mature newborn infants. The reason for diminished clot retraction is thought to be a high number of young platelets among the whole population of blood platelets.