Lipopolysaccharide is the major surface antigen of Leptospira. Variation in LPS structure is the basis for the more than 200 serovars that have been identified. Despite the importance of this antigen in immunity and diagnostics, there is relatively little known about the genetics and chemistry of leptospiral LPS, as compared to some members of the Enterobacteriaceae. The nucleotide sequence of the locus encoding enzymes for the biosynthesis of the O-antigen component of leptospiral LPS (rfb locus) has been determined for three serovars namely, L. interrogans serovar Pomona, L. interrogans serovar Hardjo subtype Hardjoprajitno and L. borgpetersenii serovar Hardjo subtype Hardjobovis. In the absence of data relating to the chemical structure or genetic tools to construct isogenic mutants in Leptospira, similarity analysis has been used to provide insight into the mechanisms by which the leptospiral O-antigen is assembled by comparison with characterized systems from other bacteria. In addition, comparison of the gene layout in each of the serovars provides an indication of the genetic basis for serovar diversity.