SPAAT (short piece of a 1 -antitrypsin [AAT]), the 44-residue C-terminal peptide of AAT, was originally isolated from human placenta [Niemann et al. (1992): Matrix 12:233-241]. It was shown to be a competitive inhibitor of serine proteases [Niemann et al. (in press): Biochem Biophys Acta]. The binding of SPAAT to one or more proteins of the extracellular matrix (ECM) was initially suggested on the basis of its recovery from tissue residues following a series of extractions designed to remove easily solubilized proteins [Niemann et al. (1992): Matrix 12:233-241]. Our binding studies with the model ECMs, Matrigel and Amgel, suggested that SPAAT might be bound by a specific collagen type as well as one or more non-collagenous ECM proteins. Individual ECM components were screened for their ability to bind SPAAT. When the four commonly occurring fiber-forming collagens (types I, II, III, and V) were evaluated, type III was found to be preferred. In addition, although SPAAT bound to preformed type III collagen fibers in a concentration dependent fashion, it did not bind to type III collagen molecules undergoing fibril formation. This is consistent with a physiological mode of interaction between SPAAT and type III collagen in vivo. Of the non-collagenous ECM macromolecules (laminin-1, fibronectin, entactin, and heparan sulfate) tested, laminin-1 was preferred. The binding of radiolabelled SPAAT to type III collagen and laminin-1 was competitively inhibited by unlabelled SPAAT as well as an unrelated protein, human serum albumin (HSA), to establish binding specificity. The kinetics of the release of the bound radiolabelled SPAAT were also examined to substantiate the non-covalent and reversible nature of this association. These results support the view that susceptible proteins of the ECM may actually be coated with SPAAT in vivo, possibly affording protection against inappropriate protease digestion.