In fish vaccinology, the secondary antibodies against fish immunoglobulins (Igs) are necessary to measure specific humoral immune responses in immunized fish. In the present study, polyclonal antiserum against olive flounder (Paralichthys olivaceus) IgM heavy chain was generated by intramuscular immunization of rabbit with Escherichia coli/eukaryotic shuttle vector containing open reading frame (ORF) of olive flounder IgM heavy chain. Western blot analysis demonstrated the specific activity of the rabbit antiserum with reduced olive flounder serum H chain at dilutions up to 1:1000. Titer of immunized rabbit serum against olive flounder serum was significantly higher than that of pre-immunized rabbit serum when determined by ELISA.