Degradation products having various grades of molecular weight were derived from fibrinogen by plasmin action. A highly purified anticoagulant active substance was obtained by the combined use of ammonium sulfate fractionation and column chromatography on DEAE-cellulose. This sub stance has the ability to inhibit completely the clotting of whole blood in a concentration of 1-1.5%, to prolong the thrombin clotting time, and to produce a macroscopically and interference-phase-contrast-microscopically abnormal clot structure. Positive eephalin-cholesterol flocculation test was also observed by adding the digest products to serum.