Eight monoclonal antibodies (MAbs) were produced against lipooligosaccharides (LOS) of Neisseria meningitidis by immunizing mice with purified LOS from a group A meningococcal strain A1. Two of the MAbs, 4385G7 and 4387A5 (isotypes IgG2a and IgG2b), reacted only with the purified LOS of strain A1, but not with 12 known purified LOS antigens (immunotypes: L1 through L12) from prototype strains except for M978 (L8) by enzyme-linked immunosorbent assay (ELISA) and immuno dot assay; the other six MAbs (4 IgM and 2 IgG3) reacted with several or many antigens of the 12 LOS immunotypes, respectively. ELISA inhibition showed that the two specific MAbs were strongly inhibited (90%) by A1 and BB431 LOS (a group B meningococcal strain), and also by M978 LOS (60%) at 100 ug/ml of purified LOS, but not by LOS or lipopolysaccharides from all other 11 prototype strains and some other gram-negative bacteria. When compared with the L8 specific MAb, 2-1-L8, by Western blot analysis and the ELISA inhibition, the MAbs 4385G7 and 4387A5 recognized a new untyped LOS epitope (type 14) in A1 and BB431 LOS which have a single 3.6 kilodalton band on silver stained SDS-PAGE gel. These results suggested that the specific MAbs can be used for typing of this new LOS epitope in N. meningitidis.