Abstract:Knowledge about the cells and organs involved in the fish immune system is important to improve fish immunoprophylaxis. Teleosts lack bone marrow and lymph nodes and the major lymphoid organs are thymus, kidney, spleen and gutassociated lymphoid tissue (GALT). In this paper, monoclonal antibodies (mabs, 2D8 and 1H1) raised against Japanese flounder (Paralichthys olivaceus) serum IgM were applied as probes. Histological, immunocytochemical and immunohistochemical detection of immunoglobulinpositive cells in the peripheral blood cells, kidney, liverpancreas, spleen and gut associated lymphoid tissues of the Japanese flounder were successfully performed. Results showed that immunoglobulinpositive cells were found in all the sampled tissues. Immunoglobulinpositive cells with diffusely stained cytoplasmic were observed in the peripheral blood, but immunoglobulinpositive macrophages were not found. There were many macrophages, granulocytes, lymphocytes and other immunerelated cells in spleen and kidney, immunoglobulinpositive cells were mostly present around the melanomacrophage centers (MMCs) and blood vessels in the spleen; the lymphoid immunoglobulinpositive cells in the kidney were also closely associated with the MMCs and existed in groups or dispersedly. The pancreas was surrounded with liver in flounder and formed liverpancreas, immunoglobulinpositive cells were separately distributed in the hepatic tissue, but not in the pancreas tissue. Additionally, immunoglobulinpositive cells in the gut associated lymphoid tissue were rarely detected in the epithelial layer although many lymphocytes were observed, yet they were frequently present in the lamina propria in groups or separately, presumably as a part of the intestine involved in mucosal immune responses. The thymus of Japanese flounder degenerates with age and no thymus could be found about 12 months old in this study.