Anti-neutrophil antibodies (CD15) bind to a simple sugar (lactose-N-fucopentaose III, LNF III) known to be present on the chains of the adhesion molecules on neutrophils. We have demonstrated that pre-incubation of neutrophils with a CD15 antibody increases neutrophil adherence to endothelium by a neutrophil-dependent mechanism. This augmented adhesion can be inhibited by antibodies directed against the alpha and beta chain of the leukocyte function-associated antigen 1 (LFA-1) molecule. There is also some increase in surface LFA-1 expression on neutrophils after CD15 incubation, suggesting that CD15 antibodies increase neutrophil adhesion by an LFA-1-dependent mechanism. The increase in LFA-1 expression after CD15 incubation occurs in the presence of a protein synthesis inhibitor. As LFA-1 is not stored intracellularly, the increased adherence of the neutrophils, and increased LFA-1 expression on their surface, suggests the possibility that the CD15 antibodies are binding to the LNF III antigen present on the alpha and beta chains of LFA-1, producing their effect by activating the molecule, perhaps by exposing new antigen sites by a stearic effect.