Abstract A lipopolysaccharide mutant of Leptospira interrogans (LaiMut) was obtained by growth in the presence of an agglutinating monoclonal antibody (mAb) against lipopolysaccharide. Agglutination reactions with anti-lipopolysaccharide mAbs and polyclonal antibodies showed that LaiMut had lost some serogroup Icterohaemorrhagiae agglutinating epitopes. However, LaiMut displayed an increased reactivity to antisera against related serogroups, suggesting that the disruption of some lipopolysaccharide epitopes resulted in greater exposure to cross-reactive epitopes, not accessible to antibodies in the wild type (LaiWT). Comparison of the nucleotide sequences of the lipopolysaccharide loci of LaiMut and Lai wild type (LaiWT) strains showed an inframe stop mutation in the gene encoding undecaprenyl-galactosyltransferase, a protein that provides a fundamental and nonredundant function essential for lipopolysaccharide biosynthesis. Despite this, the biosynthesis of lipopolysaccharide agglutinating antigens was not abolished by the mutation. Based on the phenotype of LaiMut and analysis of the domain structure of the undecaprenyl-galactosyltransferase in relation to the mutation, we propose that the mutation results in the expression of two functional proteins in place of the undecaprenyl-galactosyltransferase. We hypothesize that the loss of coordination of the coupled function afforded by the intact dual function protein present in the parent strain results in an inefficient production of lipopolysaccharide in LaiMut.