Plasma vitamin K1 (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.