Chromatography of human myeloperoxidase (MPO) on a heparin-agarose column demonstrated a tight association of the protein with the resin. The electrophoretic mobility of mixtures of MPO and heparin in polyacrylamide gels under nondenaturing conditions was consistent with a strong interaction of the cationic enzyme with the polyanionic polysaccharide. Purified MPO prebound to bovine aorta endothelial cells (BAEC) and supplemented with hydrogen peroxide dose- and time-dependently abrogated the interaction of coagulation factor IX (FIX) with factor IX-binding protein (FIXBP) on the surface of BAEC reflecting oxidative modification of the binding protein. This inactivation of FIXBP required the presence of chloride implicating hypochlorite in the reaction. Hypochlorite and activated neutrophils exerted a similar effect. The oxidative modification of FIXBP was only partially dependent on the addition of hydrogen peroxide and was abolished by exogenous heparin which displaced MPO from the cell surface, emphasizing the functional differences between cell-bound and free enzyme.