The effect of vitamin K(V.K) administration to the mother on the value of the Hepaplastin test (HPT) was studied in 1,000 neonates. The mean value for HPT on 4th day in the breast feeding group (BF) was 42.2%, which was significantly lower than that in the breast formula feeding group (BFF). 1.6% of BF, and 0.5% of BFF showed an HPT value below 20%. Administration of V.K2 to the mother resulted in a significant increase in the HPT value (BF; 58.9%, BFF; 59.1%). None of the cases showed a HPT value below 20% after the V.K administration. 203 neonates who showed a HPT value below 40% were treated with daily doses of 4 mg V.K2. The HPT value improved to over 40% in all cases within 3 days. V.K1 and K2 concentrations in maternal serum and milk were measured by the method using high performance liquid chromatography. Mean concentrations of V.K1 and K2 on the 4th postpartum day in serum were 0.7 and 0.4 ng/ml and those in the milk were 1.4 and 0.9 ng/ml, respectively. V.K1 administration to the mother after delivery significantly increased V.K1 in serum. V.K2 administration also increased serum V.K2 levels. When V.K1 was given to the mother, both V.K1 and K2 levels in the milk increased. Administration of V.K2 resulted only in an increase in V.K2 in the milk. These results indicated that the administration of V.K to the mother improves the HPT value of the neonate and may be useful in preventing V.K deficiency in the neonate.