Encephalitogenic myelin basic protein, the antigen responsible for induction of experimental allergic encephalomyelitis in animals, induced a significant proliferative lymphocyte response in the cells of five of eight patients in the acute stages of acute disseminated encephalomyelitis, a possible clinical counterpart of experimental allergic encephalomyelitis. No significant response was noted in patients recovered from acute disseminated encephalomyelitis nor in patients with multiple sclerosis or other neurologic diseases nor in normal controls. No circulating antibody to myelin basic protein was detected in the sera of any patients or controls. These findings suggest that cell-mediated immunity to myelin basic protein is a concomitant of acute disseminated encephalomyelitis and may be important in the pathogenesis of the disease.