Cerebrospinal fluid (CSF) lymphocytes from three young patients undergoing acute exacerbations of multiple sclerosis were studied by flow cytometry. Using a new method that simultaneously measures cell-surface antigens and the cell-cycle phase, we determined that the CSF lymphocytes in these patients were activated. The majority of the activated cells expressed the OKT4 phenotype, while very few expressed the OKT8 phenotype.