Serological reactions of Pseudomonas syringae and Pseudomonas viridiflava were studied by Ouchterlony double diffusion. A total of 55 polyclonal antisera, containing anti-lipopolysaccharide (anti-LPS) precipitating antibodies, were cross-tested against antigenic suspensions of 51 strains. Twenty-three O serogroups were defined, primarily on the reaction of the type strains. Two families of O serogroups showed antigenic crossreactivities (