The interactions between cell surfaces and extracellular matrix (ECM) components are impor tant for the adhesion, locomotion, and differentia tion of cells. Fibronectin (FN), laminin (LM), fibrinogen (FG), and von Willebrand factor are among a group of large, extracellular glycoproteins that mediate such cell-matrix interactions and are important in a variety of cellular processes. Recently, thrombospondin (TSP) has been recog nized as a new member of this class of so-called adhesive glycoproteins. TSP was originally dis covered in a-granules, 1 where it was seen to be re leased from α-granules on platelet activation, 2 and was suggested to have a role in platelet aggregation. This initial speculation has been borne out, since several groups have shown that either polyclonal or monoclonal antibodies against TSP can block secre tion-dependent or secondary phase platelet aggrega tion. 3-5 This result in itself, however, does not speak directly to the mechanism of TSP involvement in this complex process, since antibodies against FG, FN, and the platelet glycoprotein IIb/IIIa (a platelet FN and FG receptor) will all block platelet aggregation. 6