Lipopolysaccharides (LPS) were isolated from the crude bacterial mass of the Pseudomonas syringae pv. maculicola IMV 381 collection culture and its virulent and avirulent subcultures isolated earlier from the heterogeneous collection culture due to its natural variability during long-term storage. The composition, immunochemical properties, and certain parameters of the biological activity of the LPS preparations obtained were studied. The structural parts of the LPS macromolecule--lipid A, the core oligosaccharide, and O-specific polysaccharide (OPS)--were isolated and characterized. The following fatty acids were identified in the lipid A composition of all cultures: 3-OH-C10:0, C12:0, 2-OH-C12:0, 3-OH-C12:0, C16:1, C16:0, C18:1, and C18:0. Glucosamine (GlcN), ethanolamine (EtN), phosphoethanolamine (EtN-P), and phosphorus (P) were revealed in the hydrophilic portion of the macromolecule. In the core portion of the LPS macromolecule, glucose (Glc), rhamnose (Rha), GlcN, galactosamine (GalN), 2-keto-3-deoxyoctulosonic acid (KDO), alanine (Ala), and P were found. The peculiarities of the structure of LPS isolated from the stable collection culture (LPS(stab)) and its virulent (LPS(vir)) and avirulent (LPS(air)) subcultures were studied. LPS(vir) and LPS(avir) were identical in the monosaccharide composition and contained as the main components L-rhamnose (L-Rha) and 3-acetamido-3,6-dideoxy-D-galactose (D-Fuc3NAc), like LPS(stab) studied earlier. The NMR spectra of LPS(vir) were identical to the spectra of LPS(stab), whose O-chain repeating unit structure was studied by us earlier, whereas LPS(avir) differed from LPS(vir) in the NMR spectrum and was identified by us as the SR form. LPS(avir) was serologically identical to LP(stab) and LPS(vir). Hence, the degree of polymerism of the LPS O-chain of P. syringae pv. maculicola IMV 381 is the main virulence factor in the infected model plants. Serological relationships were studied between P. syringae pv. maculicola IMV 381 and the strains of other pathovars with structurally similar LPS.