A method was described for the quantitation of precursor prothrombin using a chromogenic substrate S2238 and Dispholidus typus venom. Prothrombin precursor was detected in the adsorbed plasma of all 65 patients on coumadin therapy tested, but was absent in 20 controls. The technic offered a rapid and sensitive method for determining if a prolonged prothrombin time is due to vitamin K antagonism or deficiency.