Vitamin K₁ serum levels were assessed by means of an off-line multidimensional liquid chromatography in 18 mothers, shortly after delivery, and in their healthy term infants. Umbilical cord and venous blood samples were assayed up to 4 weeks of life. Concurrently, levels of coagulation factors II and X, antithrombin III and platelets were established. Although the detection limit of the assay was as low as 22 pg/ml, vitamin K₁ concentration appeared to be still beyond that level in cord blood or in newborn serum within 30 min after birth, whereas vitamin-K-dependent coagulation factors are already at a level of 40 %, without evidence for the presence of descarboxy prothrombin, in any of the investigated neonates. After 3 days, breast-fed neonates had lower vitamin K₁ levels than formula-fed infants (0.76 and 1.44 ng/ml, respectively). The levels of the vitamin-K-dependent coagulation factors II and X, however, were comparable, regardless of the kind of feeding. After 28 days, breast-fed neonates had even lower vitamin K₁ levels (0.49 ng/ml, while the formula-fed infants showed higher vitamin K₁ levels (4.45 ng/ml). But even then, the levels of vitamin-K-dependent coagulation factors II and X were comparable, regardless of the kind of feeding. From this we conclude that the serum levels of vitamin K₁ in formula-fed neonates exceed those of breast-fed infants from the moment of feeding (24 h and later) without a concomitant rise in vitamin-K-dependent coagulation factors. A relationship between vitamin K₁ levels and vitamin-K-dependent coagulation factors could not be established in healthy term breast-fed or formula-fed infants.