Rabbit plasma blood coagulation factor XIII(protransglutaminase), like the human zymogen, is a tetramer $(a_{-2}b_{-2})$ as is the enzyme$(a_{-2}b_{-2})$(transglutaminase, E.C. 2.32 13) formed by thrombin activation in the absence of $Ca^{2+}$. Transglutaminase from its zymogen, blood coagulation factor XIII, catalyzes the covalent intermolecular polymerization of fibrin through formation of ${epsilon}-({gamma}$-glutamyl) lysine crosslink. Since $a_{-2}b_{-2}+b_{-2}*{ ightleftharpoons}a_{-2}b_{-2}*+b_{-2}$ occurs both in vitro and in vivo, the half-life of $b_2$ in $a_2$-deficient rabbit is 4.5 hours. These results suggest that only tetramer form$(a_{-2}b_{-2})$ of a plasma coagulation factor XIII could sustain longer in blood plasma.