Abstract
The properties of the prephase accelerator (PPA) are indicated as well as its differentiation from other known clotting factors. PPA is either a reaction product or a degratation product. For its normal formation genuine factor IX, PTA and ? Hageman factor are necessary. The one-stage quantitative assay method for factor “IX” does not determine the factor lacking in haemophilia B. This method gives a measure of PPA. Some practical observations are made concerning the value of the one-stage assay method for factor “IX” and of the thromboplastin generation test for the diagnosis of haemophilia B.