Platelet contractile activity was evaluated by observation of tension development during isometric contraction of platelet-fibrin clots. Cylindrical clots were made with platelet-rich plasma obtained from cord blood or from adult controls. These clots were allowed to contract isometrically at 37 degrees C while attached to a transducer to record tension development. The rate of tension development was dependent on platelet concentration but was equivalent for neonatal and adult platelet clots. Although abnormalities in neonatal platelet aggregation and secretion have been well documented the platelet functions required for clot contraction such as fibrin binding and actin-myosin interaction appear to be intact in neonatal platelets.