Upon activation with various noncytokine stimuli, polymorphonuclear leukocytes (PMNs) mobilize intracellular sialidase to the plasma membrane, where the sialidase releases sialic acid from the cell surface. This desialylation enhances PMN adherence, spreading, deformability, and motility, functions critical to diapedesis. We now have examined the role of sialidase activity in PMN adhesion to and migration across the endothelium in vivo. A polyclonal antibody prepared against Clostridium perfringens neuraminidase 1) detected surface expression of sialidase on human PMNs stimulated with IL-8 in vitro and on murine PMNs stimulated in vivo, but not on that of unstimulated cells, 2) recognized proteins in human PMN lysates and granule preparations that were not detected by preimmune antibody, 3) inhibited bacterial neuraminidase and human PMN sialidase activities in vitro, and 4) inhibited both pulmonary leukostasis in mice systemically infused with cobra venom factor and intrapulmonary transendothelial migration of PMNs into the bronchoalveolar compartment of mice intranasally challenged with interleukin-8. We conclude that the chemokine interleukin-8, like other PMN agonists, induces the translocation of sialidase to the PMN surface and that surface expression of this sialidase is a prerequisite to PMN recruitment in vivo. The ability of antibodies raised against a prokaryotic neuraminidase to recognize eukaryotic sialidase extends the concept of the neuraminidase superfamily to mammalian enzymes. Inhibition of mobilized endogenous sialidase may provide a novel strategy for limiting the inflammatory response.