Abstract.The protein composition of 12 different prothrombin complex concentrates, including 2 purified factor IX concentrates and 2 activated fractions, was evaluated. There is a clear difference between ion exchanger‐prepared fractions and those obtained by adsorption onto calcium phosphate. The former contain high molecular weight kininogen and complement components, the latter only trace amounts of these proteins but relatively high quantities of the pro‐ and even partially activated enzymes of the contact phase. Calcium phosphate adsorbed preparations contain less VIII:CAg than the DEAE‐Sephadex‐prepared fractions. The inorganic adsorbent showed higher affinity for IgG than the ion exc