Éva Katona, Krisztina Pénzes, Andrea Csapó, Ferenc Fazakas, Miklós L. Udvardy, Zsuzsa Bagoly, Zsuzsanna Z. Orosz, and László Muszbek Abstract Coagulation factor XIII (FXIII) is a heterotetramer consisting of 2 catalytic A subunits (FXIII–A2) and 2 protective/inhibitory B subunits (FXIII–B2). FXIII–B, a mosaic protein consisting of 10 sushi domains, significantly prolongs the lifespan of catalytic subunits in the circulation and prevents their slow progressive activation in plasmatic conditions. In this study, the biochemistry of the interaction between the 2 FXIII subunits was investigated. Using a surface plasmon resonance technique and an enzyme–linked immunosorbent assay–type binding assay, the equilibrium dissociation constant (Kd) for the interaction was established in the range of 10−10 M. Based on the measured Kd, it was calculated that in plasma approximately 1% of FXIII–A2 should be in free form. This value was confirmed experimentally by measuring FXIII–A2 in plasma samples immunodepleted of FXIII–A2B2. Free plasma FXIII–A2 is functionally active, and when activated by thrombin and Ca2+, it can cross–link fibrin. In cerebrospinal fluid and tears with much lower FXIII subunit concentrations, >80% of FXIII–A2 existed in free form. A monoclonal anti–FXIII–B antibody that prevented the interaction between the 2 subunits reacted with the recombinant combined first and second sushi domains of FXIII–B, and its epitope was localized to the peptide spanning positions 96 to 103 in the second sushi domain.