Cerebrospinal fluid myelin basic protein and cerebrospinal fluid and peripheral blood T-cell subsets have been studied in patients with multiple sclerosis and other inflammatory and non-inflammatory nervous system diseases. These biological parameters have been correlated with clinical disease activity. No changes in peripheral blood T-cell subsets were seen in multiple sclerosis patients. Low cerebrospinal fluid T8+ cells occurred only in multiple sclerosis, while high cerebrospinal fluid T4+ cells were detected both in clinically active multiple sclerosis and in inflammatory nervous system diseases. A close relationship was found between cerebrospinal fluid T4/T8 ratio and myelin basic protein in relapsing multiple sclerosis patients.