The interaction of rabbit alveolar macrophages with purified homologous fibronectin bound to gelatin-Sepharose granules was studied. Gelatin-Sepharose, albumin-Sepharose and pure Sepharose 4B granules served as control. It is found that the macrophages are able to attach to fibronectin-coated surface and that the attachment is strongly dependent upon Ca2+ and, especially, upon Mg2+. Heparin partly inhibited the cell-fibronectin interaction. The gelatinized surface (i. e. Gelatin-Sepharose) also caused adhesion of macrophages but this process was, in contrast to their binding to fibronectin, enhanced by heparin. The trypsinization of cells completely inhibited both types of interaction. The data suggest that fibronectin and receptors for fibronectin (FN-receptors) may be present simultaneously on the surface of alveolar macrophages. A significant role that fibronectin plays in mononuclear phagocytosis is pointed out, and some biological properties of alveolar macrophages are explained taking into account the existence of FN-receptors.