Cerebrospinal fluid (CSF) and serum samples from six patients with enterovirus infections were investigated by isoelectric focusing (IEF) and affinity-mediated immunoblot AMI) for the clonal distribution of entervirus-specific antibodies. In two patients with either acute meningitis or encephalitis and in one patient with a relapse of multiple sclerosis, oligoclonal IgG bands specific for enteroviruses were found predominantly in the CSF, revealing intrathecal synthesis of these antibodies. In three other patients with neurological symptoms probably unrelated to a current enterovirus infection, IEF and AMI disclosed nearly identical patterns of coxsackievirus-B-specific oligoclonal bands in the CSF and serum, indicating diffusion of these antibodies from the serum into the CSF. Although the number of patients in this study is small, the results suggest that intrathecally synthesized enterovirus-specific antibodies may be used as a means of identifying an enterovirus infection of the CNS.