Prothrombin of newborns was investigated by means of the quantitative immunoelectrophoresis and the two-dimensional immunoelectrophoresis. The plasma of 10 of 24 healthy term newborns and of 5 preterm infants taken immediately after birth showed one prothrombin precipitate in the α-2-β-globulin position when a buffer containing 25 mM calcium lactate was used. No differences in the mobility and form of the precipitate could be observed as compared to healthy adults. The plasma of the other 14 term newborns and one small-for-date infant showed two precipitates, one in the α-2-β-globulin position and a second in the α-1-globulin position. Twenty-four hr after the injection of vitamin K, the precipitate in α-1-globulin position was no longer evident. When a buffer without calcium was used in the two-dimensional immunoelectrophoresis, only one precipitate in the α-1-globulin position was found in all newborns. In the quantitative immunologic determination, the prothrombin was found to be decreased as compared to that of adults. The quantitative immunologic determination and the activity assay gave equal results in those infants who had only one precipitate in the two dimensional immunoelectrophoresis. In those newborns, who had two precipitates, the quantitative immunologic determination gave higher results than the measurement of the activity assay.From this study, no evidence of differences between the prothrombin of newborns and that of adults could be derived. Vitamin K deficiency, as judged from the appearance of a second peak in α-1-globulin position in the two dimensional immunoelectrophoresis was a frequent finding in term newborns, even on the 1st day of life.SpeculationVitamin K deficiency is not only an acquired disease in newborns due to an inadequate intake of vitamin K with certain formulas during the 1st days of life, but is also a disease that may exist in term newborns, after a normal pregnancy, before feeding. The significance of an efficient diet of the mother or of vitamin K supplementation to the mother remains to be elucidated.