Leukocyte cell surface sialyl Lewis x (sLe(x)) and related epitopes play an important role in cell rolling and adhesion during diapedesis via interaction with E-selectin. Here, we present evidence that Mac-1 (CD11b/CD18, CR-3) is a major neutrophil glycoprotein decorated with sLe(x) and ligation of these carbohydrate moieties by anti-sLe(x) antibody significantly impairs neutrophil functions. First, Western blot analysis shows that both CD11b and CD18 subunit of purified Mac-1 are decorated with sLe(x) moieties. A significant co-localization of CD11b and sLe(x) moieties is observed at neutrophil secondary granules. With stimulation of formyl-Met-Leu-Phe (fMLP), neutrophil surface labeling with anti-sLe(x) antibody follows an identical up-regulation pattern of Mac-1. Second, protein-binding assays indicate that sLe(x) moieties on Mac-1 are critical for binding interaction of Mac-1 to E-selectin. Removal of sLe(x) moieties completely abolishes Mac-1-E-selectin binding. Finally, ligation of Mac-1 sLe(x) by anti-sLe(x) antibody induces a significant degranulation of neutrophil secondary granules at the absence of chemoattractant stimulation. This "dysregulated" degranulation induced by anti-sLe(x) antibody strongly inhibits neutrophil transmigration in response to fMLP. In summary, Mac-1 sLe(x) moieties play a critical role in regulating beta(2) integrin functions during neutrophil transmigration and degranulation.