Thrombospondin (TSP) is released from alpha granules of activated platelets, binds to platelet surfaces, and copolymerizes with fibrin. In the present experiments, we investigated the action of factor XIIIa (plasma transglutaminase) on TSP. Factor XIIIa catalyzed incorporation of [14C]putrescine into soluble TSP and ligation of TSP to itself and to fibrin intermediates. Proteolytic digestion of [14C]putrescine-labeled TSP with trypsin or thrombin yielded a labeled disulfide-bonded core of 90 or 120-130 kilodalton (kDa) subunits, labeled fragments of less than 10 kDa, and an unlabeled 30-kDa heparin-binding fragment, indicating the presence of multiple factor XIIIa reactive glutaminyl residues located in several domains of the molecule. TSP became ligated in fibrin clots formed from amidinated fibrinogen, i.e., fibrin that could not contribute lysyl residues to factor IIIa catalyzed cross-links. The disulfide-bonded core of TSP formed upon thrombin digestion copolymerized with fibrin as efficiently as intact TSP. However, a lower proportion of the disulfide-bonded core became ligated. These results indicate that TSP, both in clots and in solution, contributes glutaminyl and lysyl residues to factor XIIIa catalyzed ligation. Cross-linking may be important in stabilizing interactions among TSP, fibrinogen, or fibrin and other molecules in hemostatic plugs.