Meningococcal antigen was measured by countercurrent immunoelectrophoresis in the blood and cerebrospinal fluid of 200 patients with group A meningococcal meningitis. Antigen was detected in the blood of 27 (13.5 per cent) patients. These patients had a worse prognosis and a higher incidence of allergic complications, such as arthritis and vasculitis, about 5 days after the start of antibiotic treatment. Antigen was found in the CSF of 129 (67.5 per cent) patients); antigen often persisted in the cerebrospinal fluid despite antibiotic treatment before admission. A combination of immunoelectrophoresis and routine bacteriologic study was used in the diagnosis of 162 (84.8 per cent) patients with meningococcal meningitis. High levels of antigen and a slow antigen disappearance were associated with neurologic damage. The antigen is stable and may be detected from specimens of cerebrospinal fluid dried on filter paper.