Nitric oxide is now universally recognized as an extracellular signaling molecule.Nitric oxide,produced in one cell,diffuses across the extracellular space and acts with targets in an adjoining cell.In this study,we present proof that hydrogen peroxide-like nitric oxide-acts as a true first (intercellular) messenger for a multitude of pro-inflammatory ligands.RAW 264.7 macrophages were activated with three different ligands,lipopolysaccharide,interferon-gamma or advanced glycation end products in the presence of increasing concentrations of (hydrogen peroxide scavenging) catalase.As inflammatory readouts,nitric oxide and tumor necrosis factor were determined.We hypothesize that hydrogen peroxide travels between cells propagating the signal,then a certain percentage of the readout should be inhibited by catalase in a concentration-dependent manner.The experiment showed concentration-dependent inhibition of nitric oxide and tumor necrosis factor-α production in response to all three ligands/ligand combinations (interferon-gamma,lipopolysaccharide,and chicken egg albumin-derived advanced glycation end product) in the presence of increasing concentration of catalase.For example,catalase inhibited 100％ of nitric oxide and 40％ of tumor necrosis factor-α production at its highest concentration.Our results suggest that hydrogen peroxide travels through cell membranes into the extracellular space and enters and activates adjacent cells.Like nitric oxide,we suggest that it is a ubiquitous first messenger,able to transmit cell-to-cell pro-inflammatory signals such as nitric oxide and tumor necrosis factor-c.In a therapeutic setting,our data suggest that compounds acting as hydrogen peroxide scavengers might not even need to enter the cell to act as anti-inflammatory drugs.