Abnormal, biologically inactive forms of prothrombin have previously been shown to appear in the plasma of cows or humans given coumarin anticoagulants. We have previously shown that a protein with similar properties increases in the liver of rats given these vitamin K antagonists, and have postulated that this protein represents the liver precursor to plasma prothrombin. Eight species, rats, mice, guinea pigs, hamsters, rabbits, calves, dogs, and chickens, have now been surveyed for both plasma abnormal prothrombin and liver precursor activity. Large amounts of plasma abnormal prothrombin were found in the bovine, substantial amounts were seen in the chick, and small amounts in rat and mouse plasma. With the exception of the bovine, all anticoagulant treated animals showed elevated levels of liver precursor activity in microsomal preparations. The relationship of these observations to the mechanism of action of vitamin K is discussed.