alpha M beta 2, a beta 2 integrin expressed on cells of myelomonocytic differentiation, functions as a receptor for factor X (X) and fibrinogen (Fg) and participates in leukocyte adhesion to vascular endothelium. Acquisition of high affinity ligand binding has been suggested to result from a conformational change of alpha M beta 2 in response to agonist-induced leukocyte stimulation. We now describe mAb 7A10, which preferentially binds to alpha M beta 2 on activated cells and reports the functional activation of this integrin. Agonist-stimulated monocytes and monocytic cells, but not resting cells, maximally expressed the 7A10 neoepitope, whereas expression of other selected alpha M beta 2 epitopes remained unchanged. The neoepitope was elicited equally by exposure of cells to either ADP or FMLP and did not require divalent cations for expression. Saturation of the 7A10 neoepitope by this Ab on stimulated THP-1 cells inhibited both X and Fg binding and abolished the alpha M beta 2-driven cellular coagulant response. Stimulated monocytic cells, which bound X and/or Fg, exhibited a sustained adhesion to unstimulated endothelial cell monolayers and Ab 7A10 inhibited this sustained adhesion. We conclude that activated conformers of alpha M beta 2 mediate X and Fg binding, that assembly of either X or Fg or both on alpha M beta 2 mediates leukocyte adhesion to unstimulated endothelium through sparse ICAM-1, and that mAb 7A10 can report and functionally inhibit this pathway of leukocyte adhesion.