Medullasin activity in mature granulocytes was measured in the blood of 22 patients with multiple sclerosis (definite type, 16; probable type, 6). The activity was elevated in every patient in relapse; it decreased to nearly normal levels at the beginning of improvement and further decreased to normal levels with remission. Serial determinations of the level of medullasin activity in 3 patients revealed that activity increased several days before the onset of acute exacerbation. Medullasin activity level in mature granulocytes obtained from patients with neurological diseases other than multiple sclerosis was largely within the normal range, except in 2 patients with spinocerebellar degeneration. Measurement of medullasin activity in mature granulocytes may become useful in both diagnosis and evaluation of multiple sclerosis.