One hundred and eight multiple sclerosis (MS) patients and 108 matched controls were studied for antibody levels and cellular immune responses to several viruses. There were significant increases in the mean titers of complement fixation (CF) or hemagglutination inhibition (HI), and complement-mediated cytotoxicity (CMC) tests for measles antibodies in MS patients; there was no increase in antibody titers to herpesviruses 1 and 2, or cytomegalovirus (CMV). The direct migration inhibition (DMI) tests showed no difference between MS patients and controls for measles, CMV, herpesviruses 1 and 2, or vaccinia virus antigens. Lymphocyte-mediated cytotoxicity (LMC) tests showed no difference between patients and controls, using cultures infected with measles and CMV viruses. In a study of stimulation or blocking of the LMC response by serum or cerebrospinal fluid (CSF), no effect was found. Therefore, increased levels of measles antibody in serum were again demonstrated in MS patients, but there was no difference in these patients' cellular immunity to measles virus versus that of the controls, and there was no abnormality of cellular immunity against the other viruses tested.