Proton nuclear magnetic resonance (1H NMR) and fast atom bombardment mass spectrometric analyses were performed on enzymatically derived oligosaccharides from the acidic excreted polysaccharides (EPS) from representative bacterial strains of the pea-nodulating symbiont, Rhizobium leguminosarum (128C53, 128C63, and 300) and the clover-nodulating symbiont, Rhizobium trifolii (NA-30, ANU843, 0403, TA-1, LPR5035, USDA20.102, and 4S). The results revealed structural similarities and differences between EPS of these two species. Octasaccharide units containing galactose, glucuronic acid, alpha-L-threo-hex-4-enopyranosyluronic acid, and glucose in a molar ratio of 1:1:1:5 were obtained from the EPS of the three R. leguminosarum strains and had the same primary glycosyl sequence and location of pyruvate, acetate, and 3-hydroxybutyrate substituents. About 80% of the galactose residues were acylated with 3-hydroxybutyrate, and there were two acetyl groups per repeating unit distributed between the 2 glucose residues of the main chain-derived sequence of the octasaccharides. In contrast, the R. trifolii strains had varied EPS structures, each of which differed from the common R. leguminosarum EPS structure. The EPS from one group of R. trifolii strains (0403 and LPR5035) most closely resembled the R. leguminosarum EPS but differed in that a lower number of galactose and glucose residues were substituted by 3-hydroxybutyryl and acetyl groups, respectively. The EPS from a second group of R. trifolii strains (ANU843, TA-1, and NA-30) was even more different than the R. leguminosarum EPS. These R. trifolii octasaccharides bore a single acetyl group on O-3 of the glucuronic acid residue. In addition, the level of acylation by 3-hydroxybutyryl groups was 50% of that present in the R. leguminosarum EPS. The remaining two strains of R. trifolii (USDA20.102 and 4S) had very different patterns of acylation to each other and to all of the other strains. The EPS from strain USDA20.102 practically lacked 3-hydroxybutyryl groups and had a unique degree and pattern of acetylation. The oligomers from the EPS of R. trifolii strain 4S completely lacked 3-hydroxybutyryl groups and galactose. The latter EPS contained only one O-1-carboxyethylidene group and had a different degree and pattern of acetylation. Interestingly, these two latter strains differ from the other R. trifolii strains in nodulation rates on rare clover species in the Trifolium cross-inoculation group. Thus, we define several groups of R. trifolii based upon their EPS structures and establish their similarities and distinct differences with the EPS of R. leguminosarum.(ABSTRACT TRUNCATED AT 400 WORDS)