The cell responses to different polyclonal lymphocyte activating substances were studied in 15 patients with aseptic meningitis by analysis of lymphocyte proliferation. thein vitroresponse to phytohaemagglutinin (PHA), concanavalin A (Con A) and poke weed mitogen (PWM) was investigated in blood and CSF lymphocytes simultaneously. the blood lymphocytes responded stronger than the corresponding number of CSF lymphocytes. These cells were, however, always capable of a response, suggesting that they were immunologically competent. No predominance of either PHA or Con A populations of T lymphocytes could be demonstrated in CSF compared to blood by this technique. When culture conditions were changed from the optimal cell or mitogen concentrations, the decrease in proliferation of the CSF lymphocytes was more pronounced than that of blood lymphocytes.