Objective: Myeloperoxidase (MPO), a cationic heme protein, which is abundantly stored in polymorphonuclear neutrophils (PMN), avidly binds to vascular matrix proteins, endothelial cells and the neutrophil membrane itself. Apart from its antimicrobial role during infection, MPO can modulate neutrophil function in-vitro by binding to Mac-1 integrin leading to activation and adhesion of the cells. Given the fact that activation and adhesion of PMN are a prerequisite for subsequent tissue infiltration in inflammation, we investigated whether MPO also has a role during neutrophil recruitment in-vivo. Methods: Intravital microscopy was performed in the mouse cremaster muscle model allowing direct observation of the PMN recruitment process. For evaluation of extravasation, Giemsa-stained cremaster muscle whole mounts were studied. Results: Our experiments revealed that MPO-/- mice show a significant reduction of PMN recruitment into the TNF-a stimulated cremaster muscle compared to wild type (WT) mice. Systemic injection of active MPO as well as catalytically inactive MPO into WT-mice led to a prompt and significant increase in PMN adhesion in cremaster muscle venules compared to saline-treated animals. When injected intrascrotally, MPO provoked a profound inflammatory response with strong induction of PMN adhesion and extravasation in WT-mice, which even outnumbered PMN recruitment observed after stimulation with TNF-a. This effect was also seen with inactive MPO, but could be almost completely prevented by pretreatment with a blocking antibody directed against Mac-1 (CD11b/CD18), while blockade of LFA-1 (CD11a/CD18) had no consequence. Of note, following intrascrotal injection, MPO was relocated into the lumen of cremaster muscle vessels as seen by immunostaining. Conclusion: In summary, MPO is a strong Mac-1 dependent mediator of PMN recruitment in-vivo irrespective of its catalytic activity, thus displaying a yet unappreciated function during inflammation.