The value of cerebrospinal fluid C-reactive protein (CSF CRP) determination as a diagnostic aid in infective meningitis has been investigated in four groups of children. In a "no meningitis" group of 10 children, a median CSF CRP value of 0.08 micrograms/ml was obtained (range 0 to 0.31 micrograms/ml); in a viral meningitis group of 21 children a median value of 0.01 micrograms/ml (range 0 to 3.06 micrograms/ml); in a bacterial meningitis group of 27 children a median value of 9.6 micrograms/ml (range 0 to 31.5 micrograms/ml); and in a tuberculous meningitis group of 18 children a median value of 0.29 micrograms/ml (range 0 to 4.9 micrograms/ml). CSF CRP values in the bacterial meningitis group differed significantly from those of each of the other groups (P less than 0.01), but considerable overlap between the groups detracted from the diagnostic value of the test. In six patients with bacterial meningitis with ambiguous conventional CSF chemistry results, normal CSF CRP values were found. Simultaneous serum CRP was determined in nine patients with tuberculous meningitis and 11 with bacterial meningitis, and the CRP response in both the serum and CSF appears subdued in tuberculous meningitis in comparison with bacterial meningitis. CSF CRP and total protein values were determined intermittently during a 24-hour period in ventricular CSF from two children with tuberculous meningitis who underwent temporary direct ventricular drainage. A considerable and apparently parallel diurnal variation in both values was seen. CSF CRP values have limited application in the etiologic diagnosis of meningitis.