The invention discloses a vaccine which comprises lipooligosaccharide (LOS) isolated from N. meningitidis and conjugated to a carrier protein. The invention also discloses a method of making the acellular vaccine. The method consists of two main steps. In the first step the lipooligosaccharide (LOS), chosen so it does not contain the lacto-N-neotetraose human antigen (LNnT), is detoxified by a novel procedure which uses hydrazine to remove the O-linked fatty acids. In the second step, the detoxified LOS (dLOS) is covalently conjugated to a carrier protein such as Tetanus Toxoid (TT). The dLOS produced in step 1 is 10,000 fold less toxic than the parent LOS. The conjugate vaccine exhibited a high level of immunogenicity as evidenced by the high titer of IgG antibody to native LOS, obtained in mice and rabbits. The rabbit antisera produced by the conjugate vaccine of one N.meningitidis strain (strain 7880, A,L10) exhibited bactericidal activity and cross reactivity with heterologous N. meningitidis strains. A conjugate vaccine made in this method may be multivalent, composed of dLOSs from different strains and/or immunotypes of N. meningitidis and will thus protect against all types of N. meningitidis, including type B.