To investigate the properties of pig plasma factor XI11 (a zymogen of factor XIIIa, EC 2.3.2.13, TGase) and consequently to utilize the pig blood, factor XI11 was purified to electrophoretical homogeneity after DEAE-Sephacel chromatography. The molecular weights were 320 OOO estimated by Sepharose CL-6B and 75 OOO by SDS-PAGE, suggesting this zymogen contains four subunits with identical molecular weights of 75 OOO. This proenzyme was activated by thrombin and Ca2+. Accordingly, the purified proenzyme was identified as factor XIII. The optimal temperature for incorporating monodansylcadaverine to @-substrate was 55 "C. Factor XIIIa (activated factor XIII) was inhibited by N-ethylmaleimide, p-(chloromercuri)benzoate, and iodoacetic acid in the presence of Ca2+. The rate constants for thermal denaturation (&) at 55 OC of factor XI11 and XIIIa were 5.0 X lW5 and 0.6 X s-l, respectively. Factor XIIIa catalyzed the covalent cross-linking of myosin heavy chain. The addition of crude plasma factor XI11 (including thrombin) substantially increased the gel strength of minced mackerel.