| Summary: | Catfish is a commonly-cultivated freshwater fish in Thailand and many Southeast Asian countries. The molecular data obtained for the IgM heavy chain (IgMH) of catfish have been useful for distinguishing monoclonal antibodies (mAbs). A mAb specific to Cμ<sub>1</sub> of the IgMH of catfish (IgMHCμ<sub>1</sub> mAb) was developed in a rabbit model using sequence information from bighead catfish (<i>Clarias macrocephalus</i>). The IgMHCμ<sub>1</sub> mAb strongly recognized the IgM heavy chain of the tested catfish, namely, bighead catfish, African catfish (<i>Clarias gariepinus</i>) and their hybrid (<i>C. macrocephalus</i> × <i>C. gariepinus</i>), in immunological Western blot analysis and competitive ELISAs. Additionally, the IgMHCμ<sub>1</sub> mAb successfully recognized IgM<sup>+</sup> cells by detecting IgM molecules in both secreted and membrane-bound forms in peripheral blood leukocytes (PBLs). The IgMHCμ<sub>1</sub> mAb was further used to quantify the percentage of IgM<sup>+</sup> cells among PBLs through flow cytophotometry. The IgM<sup>+</sup> cell percentages of healthy bighead catfish, African catfish and their hybrid were 38.0–39.9%, 45.6–53.2%, and 58.7–60.0%, respectively. Furthermore, the IgMHCμ<sub>1</sub> mAb showed no cross-reactivity with the IgM of zebrafish. These findings suggest that this mAb can be used as an immunological tool for monitoring the health, immune status, and immune development of cultivated <i>Clarias</i> catfish.
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