Peripheral blood from 21 patients with active and 19 patients with stable multiple sclerosis (MS), and cerebrospinal fluid (CSF) from 7 of the patients with active disease were examined for T lymphocytes by rosette formation with sheep erythrocytes. Increased numbers of T cells were found in CSF in active disease (94.0 ± 0.7 compared to 71.3 ± 4.4% in controls). Decreased relative and absolute numbers of T cells were found in peripheral blood in active, but not in stable disease (mean 47 ± 3 and 957 ± 93%/mm3 compared to 66 ± 2 and 1,381 ± 93%/mm3 in cont