An antihaemorrhagic factor named erinacin was purified from the skeletal muscle extract of the European hedgehog, Erinaceus europaeus, by ammonium sulfate precipitation followed by various steps of ion-exchange (DEAE-cellulose), absorption chromatography (hydroxylapatite), and gel filtration (cellofine gel). A 625-fold purification was achieved with an overall yield of 19% antihaemorrhagic activity. The protein effectively inhibited the activity of Bothrops jararaca venom haemorrhagin and did not inhibit the enzymatic activity of trypsin and chymotrypsin. Erinacin is a large molecule (about 1,000,000 mol. wt). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the presence of two subunits: one with an apparent mol. wt of 35,000 forming a larger subunit (350,000) by cross-linking with disulfide bridges, and a second with a mol. wt of 39,000 without disulfides. Dissociation of erinacin into its subunits resulted in complete loss of its antihaemorrhagic activity.