Natural killer (NK) cell activity, antibody-dependent cytotoxicity (ADCC) and the effect of interferon on NK activity was investigated in multiple sclerosis (MS) patients. NK activity measured against K-562 tumour cell line was found to be significantly low in MS patients, and this was most pronounced in the group of male patients with definitive disease. The response of NK cells to interferon proved to be impaired and almost no activation could be demonstrated in response to polyinosinic-polycytidylic acid (poly I:C). Preliminary data obtained by the determination of interferon production in several definitive MS cases reflect a defect in the interferon producing capacity of lymphocytes. The possible involvement of impaired NK cell function in the aetiopathogenesis of MS is discussed.