The effect of meningococcal cell-associated sialic acid on activation of the human alternative complement pathway was examined by using a quantitative fluorescence immunoassay to assess alternative pathway-mediated C3 binding to a group B strain of Neisseria meningitidis from which graded amounts of sialic acid had been removed with neuraminidase. Using human serum absorbed with strain B16B6 (B:2a:L2,3) and chelated with 10 mM MgCl2 and 10 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, we found an increase in the amount of C3 bound by enzymatically desialylated B16B6 organisms over the amount bound by fully sialylated organisms. This increase was proportional to the amount of sialic acid cleaved from the bacteria. Enhanced C3 binding was accompanied by an increase in factor B deposition. A sialic acid-deficient mutant of strain B16B6, designated 2T4-1, bound C3 via the alternative pathway at a level equivalent to that bound by wild-type meningococci from which 88% of the sialic acid had been removed. Strain B16B6 was resistant to the alternative pathway-mediated bactericidal activity of both absorbed and hypogammaglobulinemic human sera, whereas noncapsular variant 2T4-1 was sensitive to these sera. The addition of purified immune immunoglobulin M (IgM) and IgG significantly increased the alternative pathway-mediated killing of strain B16B6 organisms. IgM mediated increased bactericidal activity without an increase in C3 or factor B deposition. In contrast, the IgG-mediated killing was associated with increased binding of C3 and factor B to the organisms. Absorption studies showed that the IgM bound to the sialic acid capsule, whereas the IgG bound to noncapsular surface antigens. We conclude from these results that the group B meningococcal sialic acid capsule inhibits activation of the alternative pathway in the nonimmune host and that both IgM and IgG, although specific for different surface antigens, are capable of augmenting the alternative pathway-mediated killing of group B meningococci.