• The newborn's fibrinolytic system is not the same as that in the adult. Hypoplasminogenemia with normal (adult) levels of α2-plasmin inhibitor and plasminogen activator inhibitor are characteristic of the newborn's lytic system. This combination suggests that the newborn may have an impaired lytic system that may explain, in part, the thrombotic events that are frequently observed. Histidine-rich glycoprotein (HRG), another fibrinolytic inhibitor, retards fibrinolysis by interfering with plasminogen's binding to fibrin. Levels of HRG have been reported to be reduced in term newborns. This finding has not been studied recently and to our knowledge, there are no reports of HRG levels in premature infants. The purpose of this study was to measure the plasma levels of HRG and plasminogen in three groups of patients: normal adults (n = 48), normal term newborns (n=43), and normal premature newborns (n = 18). The protein levels were determined by electroimmunoassay. Cross-immunoelectrophoresis was also performed for HRG. Cord blood was employed for obtaining new-born citrated plasma. The newborns had significantly lower plasminogen and HRG levels when compared with those of the adults. Also, the HRG levels of the premature newborns were lower than those of the term newborns. In conclusion, the newborns had lower levels of HRG than adults, with premature newborns having the lowest levels. This may allow for more plasminogen to be available for fibrin binding even though newborns have hypoplasminogenemia.