Abstract: Provided herein are mutant strains of Neisseria meningitidis which produce lipooligosaccharide (LOS) differing from the wild-type LOS in structure as well as the mutant LOS molecules and immunogenic compositions containing truncated LOS molecules from a Neisseria strain containing a genetically stable in a gene selected from the group consisting of genes encoding .alpha.-1,2-N-acetylglucosamine transferase and a gene homology to an Escherichia coli gene encoding 1-acyl-sn-glycerol-3-phosphate acyltransferase. N. meningitidis NMB-559 harbors an insertion mutation in the gene encoding .alpha.-1, 2-N-acetylglucosamine transferase, and N. meningitidis NMB-469 harbors an insertion mutation in an nlaB gene, the protein product of which has significant amino acid sequence homology to the E. coli gene encoding 1-acyl-sn-glycerol-3-phosphate acyltransferase.