The concentration of bilirubin in the serum in a group of 119 premature and full-term infants was studied after the administration of water-soluble vitamin K and vitamin K1. It appeared that large doses of water-soluble vitamin K produced a higher concentration of bilirubin than was found in the control infants. Vitamin K1 administered intravenously did not have a hyperbilirubinemic effect. The mean concentration of bilirubin on the fifth day for infants given 25 mg of vitamin K1 orally approximated that of the untreated control group. However, the regression line of body weight versus concentration of bilirubin for this group showed a tendency towards hyperbilirubinemia in infants of less than 2500 gm (5 lb).