AFTER years of confusion, it seems that a relatively simple pattern is emerging from present theories of blood coagulation. Its recognition is assisted by the Roman numeral terminology of the International Committee on Blood Clotting Factors, which, by displacing a profusion of synonyms, allows the basis of factual agreement to be seen. Physiological clotting seems to be initiated by contact of the blood with the ‘foreign’ surfaces presented by many substances and tissues other than normal vascular endothelium. Ratnoff et al.^1–3, Margolis^4,5, and others^6–8 have established that contact activates Factor XII (Hageman factor) perhaps by unfolding its molecule, which leads successively to the activation of Factors XI (PTA) and IX (Christmas factor). It has now been shown that this is followed by the activation of Factors VIII (Antihæmophilic factor), and X (Stuart–Prower factor)⁹, the evidence suggesting that all these reactions are enzymatic.