The effect of prophylaxis with oral or intramuscular vitamin K1 (Konakion) on the hypoprothrombinaemia and on the rate of detectable acarboxyprothrombin of full-term newborns was investigated. Factor II clotting activity, factor II activity by Echis carinatus venom, factor II protein concentration and acarboxyprothrombin were determined in four groups of breast-fed infants. In the untreated group and in the group where the babies received vitamin K1 orally at birth the factor, II clotting activity was decreased and the rate of acarboxyprothrombin positive cases was increased significantly (from 30% and 28% to 55% and 52% respectively) at the 3d and 5-7th days of age. By the other two groups where 1 mg vitamin K1 was given intramusculary or 2-3 mg vitamin K1 was given orally with the first milk-feed, the factor II clotting activity increased at 3d and 5-7th days of life. In these groups the rate of acarboxyprothrombin positive babies was reduced at 3d day of life from 36% and 35% (cord blood values) to 16% and 13% respectively, and there was found acarboxyprothrombin in none of the babies at 5-7th days of life. These findings support that vitamin K1 given orally at birth is ineffective to prevent vitamin K deficiency, but when it was given with the first feed orally to well, mature babies it seems to be enable to protect the early haemorrhagic disease of the newborns.