Surface markers of cerebrospinal fluid (CSF) cells have been studied in comparison with those of peripheral blood lymphocytes. The great majority (about 95%) of CSF cells bear surface markers of thymus-dependent lymphocytes (E rosettes with uncoated sheep red blood cells), while being devoid of bone-marrow derived cell markers (EAC rosettes with complement-coated red blood cells). In contrast with lymphocytes from peripheral blood, CSF cells form predominantly ‘early’ and ‘sphere’-shaped E rosettes, and bear receptors for the complement-fixing segment of immunoglobulins G. Treatment with concentrated CSF does not affect rosette forming ability of peripheral blood lymp