Plasminogen appears to be absent from the blood of human embryos until shortly before term. In prematures plasminogen is usually absent or values are very low. Lack of plasminogen may deprive premature infants from a protective mechanism against pulmonary hyaline membrane formation. Antiplasmin is present in the blood of fetuses from the fourth month of gestation. Levels are within the normal adult range. In prematures antiplasmin levels show greater variability, but are nevertheless significantly higher than in mature infants. The greatest variability of all groups analyzed is seen in prematures with respiratory distress syndrome. Pulmonary plasminogen activator activity has been detected in human as well as guinea pig embryos at the fourth month and sixth week of gestation respectively. A sharp increase in activity occurs before term followed by a decrease in the postnatal period. The increase before term may represent a physiologic protective mechanism against pulmonary fibrin deposition which may occur at birth.