SummaryAn investigation on the influence of a dosage of 2 mg vitamin K1in premature infantsAn investgation is described, which was designed to answer two questions: 1) whether a dosage of 2 mg vitamin K1applied to premature infants is sufficient to warrant a rapid decrease of the prothrombintime during the first days of life, and 2) whether the dosage is innocuous with regard to the bilirubin content of the blood and the possibility of nuclear jaundice.