We have developed an enzyme-linked immunosorbent assay (ELISA) in order to quantitate antimeningococcal IgM and IgG serum antibodies. The B:15 meningococcal strain was used as coating antigen, and class specific antibodies were detected by using alkaline phosphatase labelled rabbit anti-human IgM or IgG as conjugate. The specific IgG activity was higher in sera from healthy meningococcal carriers than non-carriers, but the difference was not statistically significant. Antimeningococcal IgM serum antibodies were more frequent in carriers that in non-carriers. Acute sera from 34 patients with fulminant meningococcal disease contained less specific IgG and had a higher prevalence of IgM than healthy carriers and non-carriers. By combining measurement of antimeningococcal IgG and IgM antibodies in both acute and convalescent sera 15/18 meningococcal patients demonstrated an increase in either IgG and IgM antibodies during the hospital stay, giving a sensitivity of 83%. 8/118 individuals without meningococcal disease had detectable specific IgM antibodies in their serum, giving a clinical specificity of the test of 93%. We conclude that quantitation of specific IgG antimeningococcal antibodies by a whole bacteria ELISA test may be a useful test for the study of immunity against meningococcal disease in single individuals as well as in epidemiological studies. The combined use of the IgG and IgM tests is helpful in the diagnosis of meningococcal disease when blood or cerebrospinal fluid cultures are negative.