It is a well-known fact that neonatal and infantile vitamin K deficiencies cause melena neonatorum and intracranial hemorrhagic disorders. These disorders occur more frequently in breast-fed babies than in bottle-fed babies, for reasons that are still obscure. Endogenous concentrations of K vitamins (phylloquinone and menaquinones) in human placentas and in paired specimens of maternal and umbilical cord plasma were detected by high-performance liquid chromatography with fluorometric detection and postcolumn reduction. Phylloquinone (K1) and menaquinones (MK-4, MK-6, and MK-7) were found in placentas and maternal plasma. K1 and MK-4 were found in umbilical cord plasma. The experimental subjects were 13 pregnant women with normal diets (group A) and two pregnant women with diets supplemented with fermented soybeans (group B), in which MK-7 is abundant. Samples from their placentas and maternal and umbilical cord plasma were collected just after delivery. The placentas and maternal plasma of group B contained higher concentrations of MK-7 (placentas = 10.82 ng/gm and maternal plasma = 3.55 ng/ml) than did group A (placentas = 1.08 ng/gm, maternal plasma = 0.70 ng/ml). However, MK-7 was not found in umbilical cord plasma of any of the subjects. The mean K1 values of all 15 patients in placentas and maternal and umbilical cord plasma were 1.28 ng/gm, 1.54 ng/ml, and 0.11 ng/ml, respectively; mean MK-4 values were 1.18 ng/gm, 0.05 ng/ml, and 0.04 ng/ml, respectively. A high concentration of MK-4 was detected in placentas.