Abstract 1. 1. Synthetic vitamin K compounds (menadione, synkayvite, and hykinone) given in relatively large single doses caused no demonstrable hyperprothrombinemia in rabbits, dogs, or man. 2. 2. Vitamin K did not elevate the prothrombin of one individual who has a consistent level of approximately 45 per cent. Since the patient's mother and sister likewise have the same concentration of prothrombin, it can be concluded that this trait is congenital and hereditary. The condition should therefore be called congenital hypoprothrombinemia. 3. 3. Quinine sulfate in daily doses of 0.33 Gm. caused no decrease in the prothrombin of the blood. 4. 4. The significance of hyper- and hypoprothrombinemia is discussed.