Many properties have been assigned to the procollagen and properdin (Type I) modules of thrombospondin-1 (TSP1) based on activities of large proteolytic fragments of TSP1 or peptides containing TSP1-derived sequences. To examine the activities of the modules more exactly, we expressed the first properdin module (P1); the third properdin module (P3); the first and second properdin modules (P12); the first, second, and third properdin modules (P123); and the procollagen module with the first, second, and third properdin modules (CP123) in the GELEX expression vector (GE1) using the baculovirus system. GE1 encodes the pre-pro sequence, the transglutaminase cross-linking site(s), the protease-sensitive site, and the gelatin binding domain from the amino terminus of rat fibronectin. All five recombinant proteins were expressed by insect cells, secreted into the culture medium, and purified by gelatin-agarose affinity chromatography. P123 shared with TSP1 a resistance to trypsin unless reduced and alkylated. P12/GE1, P123/GE1, and CP123/GE1 bound poorly to heparin-agarose except in the absence of sodium chloride, whereas peptides based on P2 are known to bind to heparin in up to 150 mM sodium chloride. In cross-linking experiments employing activated recombinant factor XIII and the transglutaminase cross-linking site in the fibronectin-derived sequence, P12/GE1, P123/GE1, CP123/GE1, and P3/GE1 but not P1/GE1 became incorporated into a fibrin clot more than GE1 alone. Analysis of the complex indicated that cross-linking was to the portion of the fibrin alpha-chain remaining in the D-dimer of plasmin digests. P123 also cross-linked to the Aalpha-chain of unclotted fibrinogen. P123 competed for 125I-TSP1 incorporation into the fibrin clot. P123 did not cross-link to plasminogen, histidine-rich glycoprotein, fibronectin, or plasma globulins other than fibrinogen/fibrin. These results indicate that the properdin modules of TSP1 specifically interact with fibrinogen/fibrin but not with heparin under physiologic conditions.