The monoclonal antibody BOB93 binds to the surface of apoptotic neutrophils. Neutrophils have been implicated in the pathogenenesis of a variety of inflammatory diseases including the adult respiratory distress syndrome, idiopathic pulmonary fibrosis, ulcerative colitis and rheumatoid arthritis. Although the neutrophil is a vital component of the body"s defense against infectious agents, uncontrolled release of its toxic substances may inflict "friendly fire" damage on surrounding tissue and propagate the inflammatory response, leading to scarring and tissue destruction. Apoptosis leads to recognition and safe disposal of dying cells by phagocytosis. Binding of the monoclonal antibody BOB93 to apoptotic neutrophils is dependent on the presence of sialoglycoprotein fetuin, a constituent of bovine serum. Fetuin is the antigen for BOB93, and BOB93 and fetuin form a complex in solution that is necessary and sufficient for binding to apoptotic neutrophils. The antigen recognised by BOB93 may act as a "molecular bridge" between the surface of the apoptotic neutrophil and the phagocyte to modulate apoptotic neutrophil clearance at inflammatory sites. Human macrophage phagocytosis of apoptotic neutrophils was augmented in vitro by addition of the antigen recognised by BOB93, which does not bind to other apoptotic leukocytes.