In 128 patients with multiple sclerosis (MS) and 204 blood donors (control persons) the in vitro activation of ‘Ficoll-purified’ peripheral blood T lymphocytes was measured using the 3H-thymidine incorporation rate without (spontaneous proliferation) and with mitogen addition (concanavalin A, phytohemagglutinin). Mitogen responsiveness in control persons and MS patients decreased with age, reflecting a T-lymphocyte inherent mechanism for the diminution of responsiveness. The MS patients, ranging in age between 20 and 30 years, showed decreased mitogen responsiveness and a tendency to increased spontaneous proliferation compared to the control persons. These results could be an expression of a viral infect