Intrathecally synthesized antibodies against measles and rubella were determined in 221 patients with multiple sclerosis (MS) and in 476 control cases. A local production was detectable in more than 80% of the MS cases but in none of the control group. These results show that the demonstration of intrathecally synthesized antibodies against neurotropic viruses not related with respective infections may serve as helpful test in the diagnosis of MS.