A method for measuring the proliferative response of cerebrospinal fluid (CSF) lymphocytes upon stimulation and mitogens and viral antigens has been developed. The number of lymphoid cells in DNA synthesis after in vitro stimulation is counted and the results expressed as a percentage of the total number of surviving lymphocytes. Preliminary results on four MS patients indicated that their CSF lymphocytes responded considerably more strongly to purified measles virus antigen than did corresponding peripheral blood lymphocytes (PBL). The responses to crude herpes virus antigen were at least as strong in PBL as in CSF lymphocyte cultures from the same patients. The CSF lymphocytes responded slightly stronger to phytohemagglutinin than PBL did.