To assess the presence of antiviral antibody synthesis in the cerebrospinal fluid in patients with multiple sclerosis, concurrent plasma and spinal fluid determination of antibody titers against measles, varicella-zoster, rubella, mumps, cytomegalovirus, and herpes viruses were performed in 29 samples and were compared with a control group. The study revealed an increased titre of antiviral antibodies in the spinal fluid in patients with multiple sclerosis. This increased activity was markedly significant for the varicella-zoster and cytomegalovirus in patients with clinical symptoms of multiple sclerosis. There was also and increased antibody titre against cytomegalovirus and varicella-zoster in patients with well defined illness. No antibody reaction was observed in the control group. The study of the antiviral antibody activity in the spinal fluid in patients with multiple sclerosis is useful in the follow-up control and in the diagnosis of the disorder specially in our community, where the investigation of antibodies anti cytomegalovirus appears to be the most appropriate method due to its high sensitivity and absence of false positive tests.