Vitamin K prophylaxis has been developed to prevent classic haemorrhagic disease of the newborn. Single vitamin K administration after birth has been reported to fail, resulting in late haemorrhagic disease of the newborn. The preventive effect of oral administration of vitamin K1 1 mg, repeated weekly during the first three months of life, was studied in 48 healthy breast-fed infants, by determination of thrombotest, PIVKA-II and vitamin K1 concentrations at the age of 4, 8 and 12 weeks. All infants showed normal thrombotest values and PIVKA-II was not detectable. Vitamin K1 concentrations were negatively correlated with the number of days elapsed since the most recent vitamin K administration. Six to seven days after the latest application, mean levels were 1223, 927 and 748 pg/ml at ages 4, 8 and 12 weeks, respectively. In conclusion, weekly administration of vitamin K1 1 mg offers complete protection against vitamin K deficiency and does not result in an accumulation of vitamin K1 in the blood.