Cerebrospinal fluid (CSF) lymphocytes from two patients with tuberculous meningitis proliferated stronger than the corresponding peripheral blood lymphocytes (PBL) when stimulated with tuberculin purified protein derivative (PPD) in the lymphocyte transformation test after 3 days of culture. This might indicate an accumulation of specifically primed lymphocytes within the central nervous system. CSF lymphocytes and PBL from nine of ten patients with acute aseptic meningitis investigated as controls showed no or low responses when stimulated with PPD, whereas the remaining patient displayed a significant proliferation of CSF lymphocytes, which was more pronounced than that of PBL. Stimulation with the mitogens phytohaemagglutinin, concanavalin A, and pokeweek mitogen gave lower proliferation of CSF lymphocytes compared with PBL in tuberculous and aseptic meningitis. Evaluation of the proliferative response of CSF lymphocytes compared with PBL on stimulation with PPD might be a useful complement in the diagnosis of tuberculous meningitis.