Analyses of cerebrospinal fluid (CSF) for lactic acid content and pH were performed in infants and children with different types of meningitis and in varying stages of therapy to assess the clinical usefulness of these determinations. There were significant differences in both measurements for patients with aseptic, purulent, and "partially-treated" bacterial meningitis. Those with the incompletely-treated disease had an intermediate elevation in CSF lactic acid and intermediate reduction in pH compared to those with untreated suppurative meningitis and those with the fully-treated disease, or aseptic variety. Moreover, a decreasing lactic acid level in purulent meningitis was indicative of effective therapy and resolution of infection.