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 gutassociated 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 immunoglobulinpositive cells in the peripheral blood cells, kidney, liverpancreas, spleen and gut associated lymphoid tissues of the Japanese flounder were successfully performed. Results showed that immunoglobulinpositive cells were found in all the sampled tissues. Immunoglobulinpositive cells with diffusely stained cytoplasmic were observed in the peripheral blood, but immunoglobulinpositive macrophages were not found. There were many macrophages, granulocytes, lymphocytes and other immunerelated cells in spleen and kidney, immunoglobulinpositive cells were mostly present around the melanomacrophage centers (MMCs) and blood vessels in the spleen; the lymphoid immunoglobulinpositive 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 liverpancreas, immunoglobulinpositive cells were separately distributed in the hepatic tissue, but not in the pancreas tissue. Additionally, immunoglobulinpositive 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.