HLA-DR expression on T lymphocytes in cerebrospinal fluid (CSF) from patients with multiple sclerosis (MS) and acute aseptic meningo-encephalitis (AM), and from blood only from healthy controls was examined by a new double-immunofluorescence labelling assay using species-specific second layers on prefixed cell samples. Thirteen of 16 patients with AM (81%) had an elevated percentage of DR positive T cells in CSF against only two of 20 patients with MS (10%). Our data indicate that AM, an acute infection of the central nervous system (CNS), is accompanied by accumulation in CSF of activated, DR positive T cells as a reflection of actively involved cellular immunity within the CNS, while this accumulation of DR positive T cells is not seen in MS, a chronic inflammatory CNS disease, despite some of the patients being examined during clinical exacerbations.