The blastogenic transformation of lymphocytes on exposure to antigen was used to investigate cell-mediated immune responses to viruses implicated in the pathogenesis of multiple sclerosis. In 12 patients with "early" multiple sclerosis, lymphocyte reactivity to measles virus, parainfluenza virus, and vaccinia virus did not differ significantly from reactivity in a healthy control group. However, there was a significant inverse correlation between the lymphocyte blastogenic response to measles and vaccinia viruses and the predetermined degree of disability from multiple sclerosis. A deficient cellular response to measles virus, and possibly to other common viruses, is therefore probably a consequence of the disease itself, and not a causal factor.