Catla catla is the fastest growing Indian major carp and one of the major aquaculture species in South Asia. A monoclonal antibody (MAb) designated B8 MAb was produced against nylon wool-enriched thymus mononuclear cells of C. catla. This MAb did not show reactivity with macrophage and epithelial cell lines derived from catla thymus in cellular ELISA. In flow cytometric analysis of gated lymphocytes, the percentage of B8 positive (B8+) cells in thymus (n = 10, 500-600 g) was determined to be 77.7 %. Similarly, the percentage of B8+ cells in kidney, spleen and blood (n = 5) was 15.08, 1.1 and 32.17 %, respectively. Western blotting of reduced membrane proteins showed that B8 MAb reacted with a polypeptide having a molecular weight of 168.2 kDa. In indirect immunoperoxidase test, B8+ cells appeared to be lymphoid cells with a high nucleus to cytoplasmic ratio. B8 reactive cells were densely packed in central region of thymus whereas, a few cells were found to be positive in kidney and spleen sections. B8 MAb also reacted with a significant population of lymphocytes in blood smears. Considering the economic importance of C. catla, this MAb should be a useful tool for studying immune response of this fish species.