Recently, we showed that monoclonal antibodies (mAbs) that are reactive with derivatives of polysialic acid containing de-N-acetylated neuraminic acid (Neu) residues are protective against N. meningitidis group B strains (Moe et al. 2005, Infect Immun73: 2123; Flitter et al., in preparation). In addition, we found that fully de-N-acetylated PSA (i.e. poly alpha 2,8 Neu) conjugated to tetanus toxoid (DeNAc) elicits IgM and IgG antibodies of all subclasses in mice that bind to group B strains, activate human complement deposition, are protective in an infant rat model of meningococcal bacteremia and are bactericidal against group C strains (Moe et al, in press). We show here that anti-DeNAc mAbs, DA1 and DA2 (both IgM), are reactive with polysaccharides containing Neu, bind to group B, C, W135 and Y but not X strains grown in chemically defined media (CDM). However, when the group X strain is grown in CDM supplemented with human plasma, DA2 binds. Also both mAbs mediate bactericidal activity against B, C, W135, and X strains with human complement. The results suggests that N. meningitidis express and/or acquire zwitterionic de-N-acetyl sialic acid antigens that can be the target of protective antibodies.