Cerebrospinal fluid adenosine deaminase activity was assayed in 13 cases of tuberculous meningitis. In all the cases, enzymatic activity was high but subsequently showed a progressive decrease, paralleled by clinical recovery and cere- brospinal fluid cellular and biochemical data normalization. Values scarcely higher than normal were reported in cases of bacterial and viral meningitis. In cases of various acute and chronic inflammatory, degenerative, vascular and neoplastic diseases of the nervous system, cerebrospinal fluid adenosine deaminase activity did not differ significantly from that of normal subjects. The assay of cerebrospinal fluid adenosine deaminase activity seems to be of considerable practical interest with regard to the differential aetiologic diagnosis between tuberculous and viral lymphocytic meningitis.