Cerebrospinal fluid (CSF) immunoglobulins (IgA, IgG, IgM) were measured in 107 patients with meningococcal meningitis. Levels were correlated significantly with CSF total protein, and both CSF immunoglobulin and protein increased with age. C3 was measured in the CSF of 38 patients and was also closely correlated with the CSF protein level. C3 breakdown products were found in all six CSFs tested. The CSF immunoglobulin and complement were thought to have leaked from the plasma due to inflammation of the meninges and had little value in diagnosis or prognosis.