In order to determine the distribution of sIgM in immune organs and the location of SIGM at the subcellular level in Nile tilapia,polyclonal antibody against SIGM fusion protein was prepared in a New Zealand pedigree white rabbit immunized with the purified SIGM fusion protein.For further study on the location of SIGM in the intestine,gill and spleen in Oreochromis niloticus,immunohistochemistry analysis and the immunoelectron microscopy technique were carried out using the prepared polyclonal antibody.The result of the Western blotting revealed that specific antigen-antibody reaction occurred between the antiserum and its corresponding recombinant fusion protein and the antibody titer reached 1:256 000.In the intestine and gill tissues,the positive signals existed in the epithelial cell surface,but not existed in the goblet cells and the mucous cells;while in spleen tissues,the SIGM protein was mainly distributed in some cells in this organ.The immunoelectron microscopy results showed that SIGM mainly existed in the vicinity of intestinal epithelial cell membrane and the surface membranes of the epithelial cells in intestinal microvilli.In the gill epithelial tissue,SIGM mainly existed in the part of the red blood cells and the epithelial cells which surrounded the gill filaments and gill-amella.In the spleen tissue,a large number of colloidal gold particles existed in the secretory vesicles at the internal Golgi apparatus.All these results suggested that sIgM played a critical role in the muscosal immunity of teleost.These provide insights into the roles of fish sIgM in the mucosal immunity.