The level of lactic acid in the C.S.F. has been measured by gas liquid chromatography in the initial specimens taken from 43 patients with proven bacterial meningitis and 20 patients with viral meningitis, 11 of which were proven cases. The normal level of C.S.F. lactic acid is 10–15 mg 96. In the bacterial cases it was over 30 mg % in 42 out of the 43 patients; in the viral cases, it was slightly raised in most, but is was over 30 mg % in only 1 of the 20 patients. Estimation of the lactic acid level in the C.S.F. therefore appears to be an important investigation in the differential diagnosis of bacterial from viral meningitis.