Normal newborn infants have a prolonged partial thromboplastin time compared to that of older infants or adults. This finding has been related to combined deficiencies of multiple clotting factors, with the exception of proaccelerin (factor V) and antihemophilic factor (factor VIII). The present study confirms the presence of decreased titers of Hageman factor (HF, factor XII), plasma prekallikrein, and high molecular weight kininogen during the neonatal period, as demonstrated in clotting assays; the degree of these relative deficiencies is usually such that only the low titer of HF appears to contribute significantly to the abnormally long PTT. Additionally, procoagulant titers of HF and plasma prekallikrein were relatively lower than the concentration of these factors determined immunologically. The mechanisms underlying this phenomenon are not known.