C1 inhibitor, a plasma proteinase inhibitor of the serpin superfamily involved in the regulation of complement classical pathway and intrinsic blood coagulation, has been shown to bind to several components of the extracellular matrix. These reactions may be responsible for C1 inhibitor localization in the perivascular space. In the study reported here, we have examined whether C1 inhibitor could function as a substrate for plasma (factor XIIIa) or tissue transglutaminase. We made the following observations: 1) SDS-polyacrylamide gel electrophoresis and autoradiography showed that C1 inhibitor exposed to tissue transglutaminase (but not to factor XIIIa) incorporated the radioactive amine donor substrate [(3)H]putrescine in a calcium-dependent manner; 2) the maximum stoichiometry for the uptake of [(3)H]putrescine by C1 inhibitor was 1:1; 3) proteolytic cleavage and peptide sequencing of reduced and carboxymethylated [(3)H]putrescine-C1 inhibitor identified Gln(453) (P'9) as the single amine acceptor residue; 4) studies with (125)I-labeled C1 inhibitor showed that tissue transglutaminase was also able to cross-link C1 inhibitor to immobilized fibrin; and 5) C1 inhibitor cross-linked by tissue transglutaminase to immobilized fibrin had inhibitory activity against its target enzymes. Thus, tissue transglutaminase-mediated cross-linking of C1 inhibitor to fibrin or other extracellular matrix components may serve as a mechanism for covalent serpin binding and influence local regulation of the proteolytic pathways inhibited by C1 inhibitor.