Peripheral blood lymphocytes from 30 patients with multiple sclerosis (MS) responded to low doses (i.e. 0.1--5 pg total protein) of crude MS myelin basic protein (BP) as assayed by the active E-rosette test (AER). Of the MS patients studied 20 (65%) responded to control BP. The optimum response of MS lymphocytes to MS BP was obtained at a lower concentration than their response to control BP. Thirty percent of other neurological patients (OND) were stimulated by both MS and control BP. Lymphocytes of all MS patients but none of the OND responded to partially purified protein fraction of MS brain ("peak 2"). Crossed immunoelectrophoresis revealed the presence of one common specific antigen in crude MS BP and MS "peak 2" antigens. The nature of these antigens is discussed.