A monoclonal antibody (MAb), MO15, was raised against the lipopolysaccharide antigen of an epsilon15-lysogenized serogroup E(1) Salmonella strain. The O factor 15-specific MAb MO15, together with another serogroup E-specific MAb, can differentiate among phage lysogenization variants in serogroup E salmonellae. Their epitope specificities in relation to conventional O-antigenic structures are discussed.