Cerebrospinal fluid (CSF) samples from 124 patients with different neurological diseases (35 having an ascertained diagnosis of multiple sclerosis (MS) and 20--of subacute sclerosing panencephalitis (SSPE)) were investigated by immunoelectrophoresis. The correlation between the "secretory" immunoelectrophoretic pattern of CSF proteins in MS and SSPE patients and the incidence of antimeasles HI antibodies in the CSF is analysed. The results point to a different pathogenic significance of the presence of antimeasles antibodies : a) persistent measles virus infection in SSPE and b) a particular genetic constitution, coincident with a hyperreactivity to measles virus, in MS.