Soluble interleukin-2 receptors in cerebrospinal fluid from individuals with various neurological disorders. Soluble interleukin-2 (IL-2R) levels in the cerebrospinal fluid (CSF) were studied in infectious, inflammatory, degenerative, and neoplastic disorders to evaluate their usefulness as a marker for the presence of activated T cells, thus indicating an inflammatory process. CSF from control subjects and patients with stationary, progressive, and treated multiple sclerosis (MS); aseptic meningitis; lymphoid and nonlymphoid central nervous system (CNS) tumors; Alzheimer's disease, as well as serum from MS patients and control subjects were studied for levels of soluble IL-2R. A significant increase in CSF IL-2R levels was observed in patients with MS, meningitis, and lymphoid CNS tumors; the MS group showed the highest values. CSF from individuals with Alzheimer's disease and from patients with nonlymphoid tumors did not show significantly elevated values. Serum IL-2R levels were significantly higher in MS patients than in control subjects, but there was no significant correlation between individual serum and CSF IL-2R levels. This study suggests the presence of activated T-lymphocytes in the CNS of patients with MS. 