The diagnostic value of several investigations which may demonstrate bacteria or bacterial products in cerebrospinal fluid (CSF) samples from patients with meningitis are discussed. Estimation of CSF lactate and lactate dehydrogenase levels was found to be of value in the differential diagnosis of viral, bacterial and fungal meningitis and the detection of endotoxin by the Limulus amoebocyte lysate test was shown to be strongly suggestive of Gram-negative meningitis. The demonstration of bacterial capsular polysaccharides in CSF by counterimmuno-electrophoresis, latex agglutination and ELISA was of value in establishing a precise aetiological diagnosis, but the usefulness of these methods was limited by the lack of general availability of specific high-potency antisera which determine the sensitivity of the procedure. These screening tests do not replace standard analysis of CSF but provide useful ancillary evidence of meningitis. Negative results obtained from screening tests should not exclude a diagnosis of bacterial meningitis and a decision to withhold treatment should only be made after all available CSF results are evaluated in conjunction with the clinical features.