Nitrotyrosine formation is a hallmark of vascular inflammation, with polymorphonuclear neutrophil–derived (PMN-derived) and monocyte-derived myeloperoxidase (MPO) being shown to catalyze this posttranslational protein modification via oxidation of nitrite (NO 2–) to nitrogen dioxide (NO 2•). Herein, we show that MPO concentrates in the subendothelial matrix of vascular tissues by a transcytotic mechanism and serves as a catalyst of ECM protein tyrosine nitration. Purified MPO and MPO released by intraluminal degranulation of activated human PMNs avidly bound to aortic endothelial cell glycosaminoglycans in both cell monolayer and isolated vessel models. Cell-bound MPO rapidly transcytosed intact endothelium and colocalized abluminally with the ECM protein fibronectin. In the presence of the substrates hydrogen peroxide (H 2 O 2) and NO 2–, cell and vessel wall–associated MPO catalyzed nitration of ECM protein tyrosine residues, with fibronectin identified as a major target protein. Both heparin and the low–molecular weight heparin enoxaparin significantly inhibited MPO binding and protein nitrotyrosine (NO 2 Tyr) formation in both cultured endothelial cells and rat aortic tissues. MPO–/–mice treated with intraperitoneal zymosan had lower hepatic NO 2 Tyr/tyrosine ratios than did zymosan-treated wild-type mice. These data indicate that MPO significantly contributes to NO 2 Tyr formation in vivo. Moreover, transcytosis of MPO, occurring independently of leukocyte emigration, confers specificity to nitration of vascular matrix proteins.