Multiple sclerosis is commonly associated with a local humoral immune response within the central nervous system. A hallmark of this intrathecal response is the presence of electrophoretically demonstrable oligoclonal bands of IgG in the cerebrospinal fluid (CSF) of up to 95% of patients. Observations indicating that a major part of the CSF IgG in some patients may represent antibodies to SV5, a simian virus closely related to human parainfluenza type 2 virus, were recently reported by Goswami et al. We have studied thirty patients with multiple sclerosis, but although we find intrathecal synthesis of IgG antibodies reacting with SV5 in seven of these, the antibodies were not associated with oligoclonal CSF IgG bands and could in each case be explained as potentially cross-reacting antibodies to other paramyxoviruses known to be human pathogens. We have therefore been unable to confirm that SV5 may be a major intrathecal immunogen in multiple sclerosis.