Ten patients with MS were studied with leukocyte migration in agarose technique to detect changes in reactivity to encephalitogenic protein in connexion with a relapse. Six showed significant reactivity within a few days after the relapse. It decreased or disappeared during the 2 weeks after the relapse, but sometimes reappeared and was found in tests performed 2-3 months later. Five patients with cerebral infarction were studied in a similar way--in three, marked reactivity was noted within a few days after the stroke; in these, reactivity decreased or disappeared in later tests. In the two other patients, reactivity appeared in the second and/or third test. The possibility of a reactivity as in epiphenomenon due to CNS tissue destruction is discussed, and the need for antigens with a more restricted specificity for such an analysis is stressed.