The potential role of IgA antibody to Neisseria meningitidis, which blocks bacteriolysis by IgG and IgM, in producing the susceptible state in military recruits, a population at greater than expected risk, was investigated in 28 patients with meningococcal disease. IgA was removed from acute-phase sera by immunoadsorption; IgG was separated by ion-exchange, and IgM by gel-filtration chromatography. The bactericidal activity of acute-phase sera, before and after removal of IgA, and of IgG and IgM fractions from the sera, was tested against each of the homologous infecting strains. Bactericidal activity, the correlate of protective immunity, was deficient in 24 of 28 unabsorbed sera, but uniformly present after the removal of IgA in a median titer of 1:16. IgM accounted for all or nearly all of the bactericidal activity. IgG was largely inactive. Susceptibility to meningococcal disease may be affected by the blocking of bactericidal IgM by circulating IgA.