IFN-γ is a major effector cytokine, produced to induce type I immune responses. It has been cloned in several fish species including zebrafish, however to date few studies have looked at IFN-γ protein expression and bioactivity in fish. Hence, the current study focused on developing a monoclonal antibody (moAb) against zfIFN-γ. We show that the zfIFN-γ moAb specifically recognises E. coli produced recombinant IFN-γ protein and zfIFN-γ produced in transfected HEK293 cells, by Western blot analysis. Next we analysed the production of the native protein following expression induced by PHA stimulation of leukocytes in vitro or antigen re-stimulation in vivo. We show the IFN-γ protein is produced as a dimer, and that a good correlation exists between transcript expression levels and protein levels.