The monoclonal antibody (MAb) against Photobacterium damselae subsp. piscicida is species-specific. The specificity of the MAb has been tested against a range of bacterial pathogens which infect fish, including a variety of Vibrio species and Photobacterium damselae subsp. damselae, and also a selection of bacteria found in the aquatic environment. The Mab is of an IgG2a isotype.