Five Japanese eels (Anguilla japonica) were examined by immunolabeling with proliferating cell nuclear antigen (PCNA) and bromodeoxyuridine (BrdU) incorporation for assessment of proliferative activities in various tissues of Japanese eels. PCNA protein was expressed in all tissues of eels, mainly in the haematopoietic tissues, especially in the anterior kidneys, an indication for the role of PCNA in haematopoiesis. Also, positive PCNA immunolabeling was frequently seen in the spleen. PCNA labeling index in kidney and spleen of Japanese eels was correlated well with BrdU incorporation which could be indicating higher proliferative activity of these organs. Absence of correlation between PCNA and BrdU in the testes may might refer to the expression of PCNA in germ and other somatic cells, while BrdU immunostaining was only noticed in S-phase dividing spermatogenic cells. Therefore, our results demonstrated over expression of PCNA in haematopoietic tissues and testes suggest the role of PCNA in haematopoiesis and spermatogenesis in Japanese eels. Moreover, PCNA and BrdU labeling indices could be a valuable approach for analyzing cell proliferation activity in the kidney and spleen of eel tissues.