We had reported the existence of 2'-5'P3A3 receptor on the peritoneal macrophages from Wistar rat. In this paper data are given to demonstrate that the ligand, for binding with 2'-5'P3A3 receptor and enhancing the phagocytosis of macrophages, must have a three-phosphate radical on its 5'-terminus and at least three adenylate residues in 2'-5'oligoadenylate. (3H)-2'-5'P3A3 can also bind on macrophages from other animals; and the binding of (3H)-2'-5'P3A3 on Hepatoma cell line 7402 is lower than that on macrophages; whereas it is very low (binding ratio less than 1%) on erythrocytes (from rabbit, rat and mouse) or human embryo lung fibroblast cells. The binding conditions of the ligand with 2'-5'P3A3 receptor have also been investigated.