The micromethod of leukocyte migration inhibition test was used to study cellular immunity in patients with multiple sclerosis (MS) to measles virus antigens and some other infectious (vaccinia virus, tuberculin) and noninfectious (brain white matter extract) antigens. In MS patients the reaction to measles antigen was weaker than in the control group, while the reactivity to the brain white matter extract was increased. As for the responses to vaccinia virus and tuberculin, these two grous did not differ statistically. The population of MS patients under study was not homogeneous in the intensity of measles cellular immunity (MCI). In the early stages of multiple sclerosis, MCI indices did not differ from those in the control group. MCI was the weakest in those subjects developing MS early in life who showed rapid worsening of the clinical status. Besides, MCI values were age-related: in older age groups they were low both in MS patients and in control subjects.