Age-matched serum and cerebrospinal fluid from 20 multiple sclerosis patients and 20 control patients with other neurological diseases were examined for antibodies to radiolabeled measles virus and canine distemper virus using an immunoprecipitation polyacrylamide gel technique. No evidence for reactivity to unique canine distemper virus-virion polypeptides in the multiple sclerosis group was obtained. Competitive binding experiments with cerebrospinal fluid using labeled and unlabeled viral antigens failed to detect preferences in binding of antibodies for canine distemper virus versus measles virus. Cerebrospinal fluid samples tested for antiviral immunoglobulin M activity using both measles and canine distemper viruses showed no activity. The results of this study failed to implicate canine distemper virus directly as a cause of multiple sclerosis.