For the purpose to clarify whether the adsorption of materials on the cell surface is the necessary requirement for phagocytosis, the phagocytosis of iron colloid particles by ascites macrophage was examined. The iron colloid particles were adsorbed on the cell surface of macrophage and taken up into vacuoles in cytoplasm. By the pretreatment of macrophage with papain, bromelin or Nagarse, macrophages failed to adsorb colloid particles on their surface and decreased in their phagocytic activity. Papain pretreatment would liberate some substance (s) responsible for adsorption of iron colloid particles from cell surface of macrophage. Ca, Mg-free medium, and pretreatment with EDTA, heparin, saponin, trypsin, chymotrypsin, neuraminidase, hyaluronidase and lysozyme had no such effects. From these results it was concluded that adsorption of material on cell surface is necessary prerequisite for phagocytosis. This requisite was discussed in respect to the mechanism of recognition of "not-self" by the cell.