Flow cytometry was adapted to measure lymphocytes in human cerebrospinal fluid (CSF). The method was sufficiently precise, reproducible and accurate despite low cell counts. In lumbar CSF of controls with 500 to 3500 (10(3)/l) leukocytes, lymphocyte counts correlated with those in corresponding venous blood: blood/CSF ratios of approximately 2000 : 1 were found for total T cells (CD3+) and CD3+ HLA-DR-, CD3+4+, CD3+8+ subsets, ratios were increased for the lymphocyte subsets CD3+ HLA-DR+ < or = CD3+16+56+ < CD16+56+3- < CD8+3- << CD19+; CD8+4+ ratio was half of CD3+ ratio. Data indicate selective barriers (blood-brain and blood-CSF barriers) to blood lymphocyte subsets which favor the transfer of T subsets. Correlation of the subset ratios to the CD3+ ratio indicates distinct barrier properties which changed differently with acute and subacute inflammations and neuroimmunological diseases of central nervous system (CNS) in lumbar or ventricular CSF, but not with simple protein barrier disturbance. HLA DR+ T ratios were higher than HLA DR- T ratios only with controls and some neuroimmunological diseases. Lymphocyte barrier characteristics were related to protein leakage situated at the same barriers, indicating for the lymphocyte subsets selective transfer routes in control subjects and non-selective routes in patients with CNS inflammation where altered ratios revealed a mixture of both routes.