The development of monoclonal antibody probes (MAbs) to bacterial and parasitic fish pathogens has provided useful tools for the rapid immuno-diagnosis and control of disease. Immunohistochemistry, IFAT and ELISA tests have been developed to detect a variety of infectious agents, including those causing bacterial kidney disease (Renibacterium salmoninarum), mycobacteriosis (Mycobacterium spp.) and proliferative kidney disease (PKX). MAb probes are also being utilised as specific markers to identify and map potential vaccine candidates for PKD. As well as illustrating the invaluable contribution which MAbs can make to the control of fish diseases, the results indicate the ephemeral nature of some antigens and the importance of pathogen culture conditions prior to the production of MAbs.