In thirty-two patients with multiple sclerosis (MS), significantly lower percentages of active T cells--that is, lymphocytes which have been incubated at 37 degrees C for 1 h before 5 min rosetting with sheep erythrocytes--were found in cerebrospinal fluid (CSF) than in blood, whereas the reverse was observed in twenty of twenty-two patients with other neurological diseases (OND). No significant difference was found between percentages of active T cells in blood in MS, OND, and healthy controls. Lymphocytes from MS CSF are extensively temperature-labile when examined under different test conditions; without incubation at 37 degrees C for 1 h, active T cell percentages in CSF of both patients with MS and OND were, in fact, higher than in peripheral blood. The mitogen response patterns of enriched active T cells and unseparated lymphocytes from peripheral blood did not discriminate between patients with MS and healthy controls. Although active T cell values have been shown to correlate with cell-mediated immunocompetence, they have not yet been defined functionally. One of the explanations for the present findings could be that lymphocytes themselves in MS patients' CSF are at least partly virus-infected.