Interaction of rabbit alveolar macrophages (AM) with homologous fibronectin bound to gelatin-sepharose granules was studied. Macrophages obtained at the height of anaphylactic shock and cells from sensitized animals after preincubation with antigen or histamine failed to adhere to the fibronectin-coated surface. AM from intact and sensitized animals were capable of adhering to the fibronectin-coated granules in a Ca2+, Mg2+-dependent manner. Pretreatment of intact cells with trypsin inhibited AM-fibronectin interaction. The data indicate that the surface of AM bears protein receptors, capable of interacting with fibronectin, which change their affinity during anaphylactic shock and under the effect of histamine. It may be thus concluded that fibronectin is involved in the function of mononuclear phagocytes in health, evidence that complements the idea of the role played by macrophages in the pathogenesis of the immediate type hypersensitivity.