White-cell count and differential blood count, total protein, lactate dehydrogenase activity (E.C. 1.1.1.27) and lactate concentration were determined in 496 CSF samples, obtained by lumbar puncture. Lactate was measured with a new enzymatic test. Reference values were determined for lactate and lactate dehydrogenase (90% limits: 1.2-2.1 mmol/l and 6-26 U/l, respectively). Lactate content proved to be best in the differential diagnosis between acute bacterial and abacterial meningitis: concentrations of 3.5 mmol/ml and above were found exclusively in bacterial meningitis. As for other diseases, markedly increased lactate concentrations were measured only in patients with brain tumour or cerebrovascular disease (up to 9.3 mmol/l). Determination of lactate concentration alone would have led to a misdiagnosis of acute bacterial meningitis in 3% of cases. If the CSF contains 3.5 mmol/l lactate or more and the leucocyte cell count is above 800/microliter, brain tumour or cerebrovascular disease can be practically excluded and the diagnosis of bacterial meningitis made with a high degree of reliability.