No antigen competition was determined between the A-layer of Aeromonas salmonicida and other bacterial cell surface antigens (constitutive and iron-regulated outer membrane proteins and lipopolysaccharide) when fish received a single or double dose of an A-layer-positive bacterin. The exposure of fish to two doses of antigen significantly enhanced antibody titres compared to those of fish receiving only one dose. Immune complexes, composed of fluorescence labelled human γ-globulin (HGG) antigen and Atlantic salmon anti-HGG immune serum, bound to a high percentage of non-fractionated PBL, sIg⁺ leucocytes (mostly B cells) and sIg^- peripheral blood leucocytes (mostly lymphocytes, neutrophils and thrombocytes), as measured by flow cytometry. These immune complex receptors are proposed to be antibody receptors. The ELISA method was investigated as a means of forming immune complexes (HGG and salmon anti-HGG) of equivalent antigen:antibody ratios, where no free antigen or antibody molecules are available. The dissociation of antibody from antigen to which it is pre-complexed in solution, and preferential binding of that antibody to solid-phase antigen, meant that the technique was unsuitable for the detection of an immune complex of equivalent antigen:antibody ratios. Immune complexes, formed in vivo by a separate injection of a low concentration of immune serum and HGG antigen, did not modulate the anti-HGG antibody responses compared to those of fish injected with HGG antigen and normal salmon serum. The in vitro complexing of specific antibody to antigen resulted in rapid clearance of the antibody from serum (within 4 weeks) in comparison to clearance of antibody alone.