PIP: Vitamin K in oral drops and intramuscular injection given at birth to Thai infants were compared to determine whether these routes and doses would influence prothrombin complex activity, mortality or morbidity at 0.5, 1 and 2 months of age. The infants were 321 normal fullterm babies born at Bangkok Adventist Hospital in 1983, exclusively breastfed during the study. Prothrombin complex (PC) was measured by the Owren capillary thrombotest method using a reagent from Nyegaard Co., Oslo. Vitamin K was given in single 1 or 2 mg oral doses, or 1 mg im, within 12 hours of delivery. Judging by the number of PC deficient children, the 1 mg im and 2 mg oral doses of vitamin K maintained clotting factors best at 2 months of age. All formulations were significantly better than no treatment at 1 month at age. No toxicity or side effects were seen. Vitamin K deficiency is a known cause of bleeding disorders, particularly fatal and handicapping intracranial hemorrhage in newborns, in developing countries where injections cannot be given by midwives. These inexpensive oral pediatric drops may provide a practical form of primary health care for routine vitamin K prophylaxis in newborns.