Plasma vitamin K₁ and proteins induced by vitamin K absence (PIVKA) were assayed simultaneously 1–4 days and 29–35 days after delivery in three groups of infants: breast-fed not receiving vitamin K at birth (n=12), bottle-fed without vitamin K administration at birth (n=7) and breast-fed receiving 1 mg vitamin K₁ administered by intramuscular injection at birth (n=13). The bottle-fed infants had a significantly higher vitamin K₁ plasma level than breast-fed infants who did not receive vitamin K₁ at birth. Extremely high levels of vitamin K were obtained 1–4 days after intramuscular administration. At the age of 1 month, breast-fed infants had the same plasma vitamin K₁ concentration whether or not they had received vitamin K₁ supplements. Decarboxy prothrombin (PIVKA-II) a reliable indicator of biochemical vitamin K deficiency, was found in 5 out of 12 breast-fed and in 2 out of 6 bottle-fed infants who had not received supplemental vitamin K₁ after birth. In a separate study, we followed up to 90 days after birth a larger group if infants. PIVKA-II was found with significantly greater frequency in breast-fed infants receiving no vitamin K than in breast-fed infants receiving 1 mg vitamin K intramuscularly at birth, or in bottle-fed infants without extra vitamin K₁. These data form a strong argument for routine vitamin K prophylaxis after birth for all breast-fed infants. The optimum dose and manner of administration require further study.