For cerebrospinal fluid (CSF) samples from 50 subjects previously selected to represent suitably a normal control group and for a methodology that may be actually used for CSF samples even with normal cell count, it is possible to characterize lymphocyte subpopulations with normal statistical distribution and low dispersion of results for all lines considered. Data (mean ± standard deviation) here estimated were (%): for B-lymphocyte subpopula-tion 7.0±1.44; for T lymphocyte subpopulation 76.0±2.92; for T-active lymphocytes 53.7±3.26; for T-avid lymphocytes 19.2±4.32 when all T subpopulation is considered, and 11.1 ±2.04 when only T-active subpopulation is studied; for T-sensitized or actually activated 22.3±3.95. These data may suggest that lymphocytes actually related to immunological activation will be properly represented by the difference between T-lymphocytes and its T-active line in each case. In every samples T and T-active lymphocytes are different subpopulations when surface receptor density is considered, by evidence of avidity for sheep erythrocytes. It is also possible that killer lymphocytes, or antibody mediated cytotoxic cells, may be related to modified lymphoid cells identified in the CSF normal cytomorphological profile.