Rhizobia are a group of nitrogen-fixing soil bacteria that establish a symbiotic relationship with leguminous plants through the formation of root nodules. Root nodule formation requires the exchange of signal molecules between the Rhizobium symbiont and the legume host. The signal molecules (Nod factors) produced by rhizobia are chito-oligosaccharides of three to five b-1,4-linked N-acetylglucosaminosyl residues, which are N-acylated on the terminal non-reducing glucosamine residue and may be further modified by the addition of another glycosyl residue or other modifying group such as sulfate, carbamyl, acetyl, and/or glycerol substitutents. The flavonoid molecules excreted by the legume plant root will induce the bacterial genes (nod genes) required for the expression of these Nod factors. The nod genes that determine host specificity dictate variations in the type of N-acyl substituent placed on the non-reducing end glucosamine and in the substituents that are present on the reducing end N-acetylglucosamine. A single species of Rhizobium can produce a variety of lipooligosaccharides with different groups (mentioned above) on their non-reducing ends and on their reducing ends. Since the Nod factors have the identical backbone structures with different substituents, mass spectrometry can be used to identify and locate the substituent groups. In the present study, we have used FAB mass spectrometry and tandem mass spectrometry to characterize the lipooligosaccharides fractionated from the cell cultures of Bradyrhizobium elkani.