We report here a novel glycosylphosphatidyl-inositol (GPI)-anchored glycoprotein on human leukocytes. Treatment of neutrophils with a mAb (3H9) to this molecule sequentially up-regulates and down-regulates β 2 integrin-dependent adhesion of these cells as well as their transendothelial migration in vitro. In addition, this mAb simultaneously modulates the avidity of β 2 integrin for its ligand, iC3b, with kinetics similar to those observed in 3H9 modulation of neutrophil adherence. This mAb also induces β 2 integrin-dependent cytoskeletal remodeling. This novel GPI-anchored protein (GPI-80) is highly homologous with Vanin-1, a recently reported GPI-anchored protein that is expressed on perivascular thymic stromal cells and is involved in thymus homing in mice. The finding that both GPI-80 and Vanin-1 are 40% homologous with human biotinidase suggests the existence of a biotinidase superfamily of molecules that may be involved in the regulation of leukocyte trafficking.