The ultimate step in the blood coagulation cascade is the formation of fibrin. Several proteins are known to bind to fibrin and may thereby change clot properties or clot function. Our previous studies identified carboxypeptidase N (CPN) as a novel plasma clot component. CPN cleaves C-terminal lysine and arginine residues from several proteins. The activity of CPN is increased upon its proteolysis by several proteases. The aim of this study is to investigate the presence of CPN in a plasma clot in more detail. Plasma clots were formed by adding thrombin, CaCl2 and aprotinin to citrated plasma. Unbound proteins were washed away and non-covalently bound proteins were extracted and analyzed with 20 gel electrophoresis and mass spectrometry. The identification of CPN as a fibrin clot-bound protein was verified using Western blotting. Clot-bound CPN consisted of the same molecular forms as CPN in plasma and its content was approximately 30 ng/ml plasma clot. Using surface plasmon resonance we showed that CPN can bind to fibrinogen as well as to fibrin. In conclusion, CPN binds to fibrinogen and is present in a fibrin clot prepared from plasma. Because CPN binds to a fibrin clot, there could be a possible role for CPN as a fibrinolysis inhibitor. (C) 2012 Elsevier Inc. All rights reserved.