Paired samples of serum and concentrated cerebrospinal fluid (CSF) from 20 multiple sclerosis (MS) patients and 10 controls with no central nervous system disease were electrofocused and analysed for IgG and measles and varicella zoster virus IgG antibodies by imprint immunofixation using IgG subclass specific monoclonal antibodies. All but one of the MS patients had intrathecally synthesized bands of oligoclonal IgG in the CSF. In fifteen of the MS patients the oligoclonal IgG bands were restricted to the IgG1 subclass. Intrathecal synthesis of both IgG1 and IgG3 bands was observed in three, and of IgG1, IgG2 and IgG3 bands in one patient. Fourteen and 15 of the MS patients displayed intrathecal synthesis of oligoclonal varicella zoster and measles virus specific IgG1 antibodies, respectively, and no serum or CSF sample contained antibodies of other IgG subclasses to these viruses. Low levels of polyclonal IgG1 antibodies to measles or varicella zoster viruses were detected in serum and CSF from the controls; none displayed evidence of intrathecal antibody synthesis. These findings are discussed in relation to theories of specific and non-specific intrathecal immune responses in MS.