The neisserial alpha-2,3-sialyltransferase, which is encoded by the lst gene, terminally links sialic acid to the lacto-N-neotetraose residue of neisserial lipooligosaccharide (LOS). We used the recently published nucleotide sequence of the neisserial lst gene to construct an isogenic serogroup B meningococcal lst mutant by insertion of a kanamycin resistance gene. The resulting lst mutant expressed the unsialylated lacto-N-neotetraose structure. Using bactericidal assays and an infant rat model of meningococcal infection, we were able to demonstrate that lst mutation, in contrast to galE mutation, which results in a truncated LOS, or to siaD mutation, which results in loss of the capsule, neither had an effect on resistance to normal human serum, nor did it impair the ability of meningococci to spread systemically in the non-immune host. The lst mutant was serum resistant despite of the fact that the central factor of complement activation, C3b, was deposited on the lst mutant as efficiently as it was on the galE mutant. Thus, the terminal sialic acid residue linked to the wild-type LOS inhibited C3b deposition on the meningocuccus. However, in contrast to the galE mutant, where C3b deposition is promoted by IgM binding, the lst mutant's surface is not a target for IgM molecules. Thus, the lacto-N-neotetraose residue of neisserial LOS alone, without the presence of terminal sialic acid, is sufficient to block IgM epitopes either on the LOS itself, or on other surface molecules. Our data provide further insight into the complex interplay of capsular and LOS sialic acids in serogroup B meningococci with host effector mechanisms, and suggest that LOS sialylation in meningococci is of a less central importance as it is in gonococci.