Plasma vitamin K] (phylloquinone) was assayed in normal adults and pregnant women at term and their babies by a method based on high-performance liquid chromatography. The mean plasma concentration in 30 healthy, fasting adults was 0·26 ng/ml (range 0·10-0·66). 8 out of 9 healthy mothers at term had a mean K₁ concentration of 0·20 ng/ml (range 0·13-0·29), but K₁ was not detected in the cord plasma of their babies. 1 mg vitamin K₁ given intravenously to 6 mothers shortly before delivery raised their plasma K₁ to 45-93 ng/ml: K₁ was then detectable in the cord plasma of 4 of the 6 infants but at a much lower concentration which did not exceed 0·14 ng/ml. The large concentration gradient between maternal and neonatal plasma suggests that vitamin K₁ does not cross the placenta readily or that the uptake by fetal plasma is low, perhaps because of low levels of a binding lipoprotein. The low levels of vitamin K in the cord plasma of the normal newborn would explain "physiological" hypoprothrombinaemia and suggest the need to reassess current clinical practice in respect of vitamin K prophylaxis in the early neonatal period.