The lipopolysaccharide (LPS) from Aeromonas salmonicida was incorporated into liposomes in order to examine the potential of these formulations to enhance the immunogenicity of LPS in rainbow trout (Oncorhynchus mykiss). Repeated intraperitoneal (i.p.) injection of free LPS resulted in the generation of anti-LPS reactivity in the serum which remained elevated for 2-4 weeks after the final injection of LPS. When administered i.p., LPS incorporated into multilamellar vesicles (MLV/LPS) or large unilamellar vesicles produced via extrusion through 200 nm pore size filters (LUVET200/LPS) prolonged the period over which serum anti-LPS antibody levels remained elevated to 6-14 weeks. Administration of two doses of either positively-or negatively-charged MLV/LPS prior to exposure to free LPS generated an anti-LPS antibody response which persisted for several weeks longer than immunisation with LPS alone. Positively-and negatively-charged LUVET200/LPS administered i.p. also resulted in extended durations of serum anti-LPS antibody responses. Injection of liposomally incorporated LPS i.p. also induced anti-lipid A antibodies in sera. The data suggests that, when administered i.p., liposomally-incorporated LPS from A. salmonicida is capable of prolonging humoral immune responses against LPS in rainbow trout. © 1997 Academic Press Limited.