A monoclonal antibody (Mab) has been raised against native thrombospondin (TSP), the endogenous lectin of human platelets, that inhibits the hemagglutination of trypsinized, glutaraldehyde-fixed human erythrocytes by purified TSP. This Mab, designated A2.5, also inhibits the agglutination of fixed, activated platelets by TSP. Mab A2.5 immunoprecipitates a 25-kilodalton (kDa) peptide from chymotryptic digests of TSP that is not disulfide bonded to any other region of the TSP molecule. This fragment represents the previously characterized heparin binding domain of TSP [Dixit, V. M., Grant, G. A., Santoro, S. A., & Frazier, W. A. (1984) J. Biol. Chem. 259, 10100-10105]. In agreement with this assignment, heparin inhibits the binding of Mab A2.5 to TSP. Another Mab, designated C6.7, also blocks TSP-mediated hemagglutination, yet has no effect on the agglutination of fixed, activated platelets by TSP. This Mab has been shown to inhibit the thrombin-stimulated aggregation of live platelets and to immunoprecipitate an 18-kDa fragment from chymotryptic digests, which is distinct from the heparin binding domain [Dixit, V. M., Haverstick, D. M., O'Rourke, K. M., Hennessy, S. W., Grant, G. A., Santoro, S. A., & Frazier, W. A. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 3472-3476].