A group of multiple sclerosis (MS) patients (n = 50), most of them in a stable or slightly progressive phase was studied together with a group of neurological control (NC) patients with CNS lesions (n = 38) comparable in age and disability and an age matched group of healthy persons (n = 46). On stimulation of lymphocytes with herpes simplex virus (HSV), cytomegalovirus (CMV) and varicella zoster virus (VZV) a significantly lower interferon production was found in MS patients compared with the NC patients and the healthy controls; when the NC patients were compared with the healthy controls a significantly lower production was also found. The lymphocyte transformation reaction in seropositive MS patients was significantly lower than that seen in the seropositive control groups when HSV was used as antigen. Lower responses, although insignificant, were found when CMV and VZV were used as antigens. There were no differences among the three groups when Con A was used as mitogen in three different concentrations. Likewise spontaneous lymphocyte transformations were equal. The lower interferon production and the lower transformation reactions in MS patients could not be correlated to age, duration of disease, disability, disease activity or impairment. No significant differences in humoral immunity to HSV, CMV and VZV were found in the three groups, a few MS patients were 'true' seronegative to HSV type 1 and 2. Suppressor cell activity induced by Con A and measles virus was comparable in the three groups, although a few MS patients in an active relapsing phase were found to have low suppressor cell activity. Natural killer cell activity against two different cell lines was found equal in the three groups, and there was no difference in the potentiating effect of interferon.