1. Coagulation Factor X was purified from human serum to apparent homogeneity in disc gel electrophoresis, sodium dodecyl sulphate-polacrylamide-gel electrophoresis, immunoelectrophoresis and analytical ultracentrifugation. The method used was a modification of that described by Gladhaug & Prydz (1970). 2. The method permits the isolation of an activated form of Factor X (Xa) which has a molecular weight of about 25000. 3. Factor Xa is a glycoprotein containing about 14% carbohydrate. A preliminary report of the amino acid composition is given.