Recently, it has been shown that CORM-released CO can suppress inflammation. In this study we assessed the effects and potential mechanisms of ruthenium-based water-soluble CO-carrier, CORM-3, in modulation of PMN inflammatory responses in an experimental model of sepsis. Sepsis in mice was induced by cecal ligation/puncture (CLP). CORM-3 (3mg/kg; iv) was administered 15 min after CLP induction. PMN accumulation in the lung (MPO assay), bronchoalveolar lavage fluid (BAL), and lung vascular permeability (protein content in BAL) were assessed 6hr later. In in vitro experiments human PMN were primed with LPS (10ng/ml) and subsequently stimulated with fMLP (100nM). PMN production of ROS (L-012/DHR123 oxidation), degranulation (release of elastase) and PMN rolling, adhesion and migration to/across human umbilical vein endothelial cells (HUVEC) were assessed in the presence or absence of CORM-3 (1-100 µM). The obtained results indicate that systemically administered CORM-3 attenuates PMN accumulation and vascular permeability in the septic lung. Surprisingly, in an in vitro experiments treatment of PMN with CORM-3 further augmented LPS/fMLP-induced ROS production and release of elastase. The latter effects, however, were accompanied by inability of PMN to mobilize elastase to the cell surface (plasma-membrane), an event required for efficient PMN transendothelial migration. CORM-3-induced decrease in cell surface levels of elastase was followed by decreased PMN rolling/adhesion to HUVEC and complete prevention of PMN migration across HUVEC. On the contrary, treating HUVEC with CORM-3 had no effect on PMN transendothelial migration. Taken together these findings indicate that in sepsis CORM3-released CO while further amplifying ROS production and degranulation of PMN concurrently reduces the levels of cell surface-bound elastase, which contributes to suppressed PMN transendothelial migration.