The value of C reactive protein measurement in the differential diagnosis of meningitis was assessed in a population where tuberculous meningitis is prevalent. C reactive protein was measured serially with a sensitive radioimmunoassay in sera from 31 children with bacterial meningitis, 15 with tuberculous meningitis (6 with miliary tuberculosis), and 28 with viral meningitis. Concentrations of C reactive protein in patients with tuberculous meningitis lay between those of patients with bacterial and viral meningitis--a finding which detracts from the virtually absolute discrimination C reactive protein measurement allows between bacterial and viral meningitis. In all but two of the patients with tuberculous meningitis, C reactive protein concentrations fell rapidly after treatment began and became normal after 10 days. This fall did not, however, exclude the development of hydrocephalus as a complication. Measurement of C reactive protein remains a useful additional parameter in the diagnosis and management of the various types of meningitis.