Summary 1. The lack of coagulation in classical haemophilia (haemophilia A) is chiefly a globulin deficiency in plasma (A.H.G. : antihaemophilic globulin). 2. The last progress in the study of haemophilia is the breakdown of haemophilia into different types : haemophilia B (Christmas Disease, P.T.C. Deficiency) and haemophilia C (P.T.A. Deficiency). This two different haemorrhagic states arc characterized by a deficiency of a serumfactor, the antihaemophilic globulin is normally present in the plasma of type A and B. 3. Thirthy-eith haemorrhagic conditions with a probable diagnosis of hae mophilia were examined : thirthy four cases were haemophiliacs of type A, and four cases were haemophiliacs of type B. Haemostasis and plasma coagulation in this four cases of Haemophilia B are studied.