The fibronectin (Fn) monomer contains two major sites of fibrin binding affinity present within the NH2-terminal and COOH-terminal domains; they consist of five (1F1-5F1) and three (10F1-12F1) consecutive type 1 modules, respectively. Recently, we have reported that the fourth and fifth type 1 module pair (4F1.5F1) of the NH2-terminal domain of fibronectin demonstrated fibrin binding ability (Williams, M. J., Phan, I., Harvery, T. S., Rostagno, A., Gold, L. I., and Campbell, I. D. (1994) J. Mol. Biol. 235, 1303-1311). In an attempt to further localize fibrin binding activity and to characterize the nature of the interaction between different type 1 modules of Fn and fibrin, we have tested a range of recombinant proteins and subtilisin generated proteolytic fragments of Fn in an enzyme-linked immunosorbent assay (ELISA) and by fibrin affinity chromatography. Of the recombinant proteins, we found that only the 4F1.5F1 exhibited significant fibrin binding activity, while 1F1, 1F1.2F1, 7F1, and 10F1 had little to no affinity for fibrin. On a molar basis, 4-5 times more 4F1.5F1 than a proteolytic fragment, corresponding to 1F1-5F1 (25.9 kDa) was required to cause 50% inhibition (IC50) of intact biotinylated Fn binding to fibrin in a competitive ELISA. This suggests that all five type 1 modules in tandem engender higher fibrin binding activity than the 4F1.5F1 alone. Furthermore, since fibrin binding activity of the intact Fn molecule was inhibited, by 70-80%, by the 4F1.5F1, the 25.9-kDa fragment, and a MoAb mapped to an epitope on the 4F1.5F1, the fibrin-binding site within the 4F1.5F1 contributes greatly to the non-covalent interaction of intact Fn with fibrin. These results provide significant insight into the Fn/fibrin interaction, a major component of the processes of wound repair and fibrin matrix assembly.