The fibrinolysin system is incomplete in newborn infants. Lack of serum plasminogen in premature newborn has an important role in the pathophysiology of the respiratory distress syndrome since alveolar fibrin deposits cannot be eliminated. Urokinase activated human plasmin has increased the survival rate of infants with respiratory distress syndrome. Plasminogen given I.V. at birth has reduced the incidence and the severity of respiratory distress syndrome, in a randomized double-blind study of 500 premature infants. Death in the plasminogen recipient group occurred only among infants born to mothers with bleeding complications of pregnancy. Plasmin inhibitors measured with a functional assay were the highest in this group of infants, serum plasminogen was the lowest; when activator and purified human plasminogen were added to the serum, fibrinolytic activity was elicited in excess of the plasminogen added. It is suggested that plasminogen and/or plasmin inhibitors may be abnormal fetal variants in infants born to mothers with bleeding complications.