Bacterial subcellular components and probiotics were successful for the stimulation of immunity and the prevention of Vibrio harveyi infections in rainbow trout, Oncorhynchus mykiss (Walbaum). Rainbow trout were immunized with whole inactivated cells of V. harveyi to obtain polyclonal antibodies against specific antigens. Western blotting showed a unique reactive band (approximately 93 kDa) between serum and bacterial proteins from outer membrane proteins (OMP) and extracellular products (ECP). Probiotics were selected according to their capability to inhibit V. harveyi. Two of these bacteria, i.e. A3-47 and A3-51, showed cross-reactivity with V. harveyi antiserum. Their OMPs and ECPs were reactive with V. harveyi antiserum in bands of approximately 93 kDa for A3-51 and higher for A3-47. In vivo tests determined that fish fed with A3-51 produced cross-reactive antibodies against V. harveyi and also, the survival of these fish infected with V. harveyi was high, being similar to the level achieved with vaccinated fish. Thus, the probiotics, when administered as live preparations, were capable of producing cross-reactive antibody against specific bacterial pathogens.