The prophylactic use of vitamin K in the newborn has been abandoned by some physicians because of conflicting reports as to its efficacy and safety. In the present study vitamin K1 was given either in doses of 5 mg. by mouth daily to the mother in the last month of pregnancy or in doses of 5 mg. intramuscularly to the infant at birth. A test series of 133 infants was treated prophylactically in this way. The contratest series consisted of 50 untreated infants. Among the 50, postcircumcision bleeding occurred 4 times and abnormal prothrombin values were observed 14 times. In the test group there was no postcircumcision bleeding, and the postnatal increase in prothrombin time was controlled without evidence of increased bilirubinemia or hemolysis.