Serum and cerebrospinal fluid (CSF) from 28 patients with multiple sclerosis (MS), 14 with other neurological diseases (OND) and 31 control subjects with tension headache were analysed for presence of IgM antibodies against measles, mumps and varicellae zoster by a specific enzyme-linked immunosorbent assay (ELISA). This technique excluded false positive reaction due to possible presence of rheumatoid factor. Twelve of the 28 patients with MS had IgM antibodies in serum and 4 in CSF, the latter always being accompanied by presence of corresponding IgM antibodies in serum. Six patients had mumps specific IgM, 5 had measles specific IgM and 3 varicellae specific IgM. In 2 patients, viral IgM antibodies were demonstrated in serum against 2 different viruses. Among the 14 OND patients, one with Wilson's disease had demonstrable serum IgM varicellae antibodies and one with radicultis had elevated serum and CSF measles and varicellae IgM antibodies. Among 31 controls, 2 had IgM antibodies in serum, one against varicellae and one against mumps. No correlations were found between viral IgM antibodies and CSF IgM index, serum IgM levels or blood-brain barrier state. Our data show that MS may be accompanied by a systemic IgM response against the 3 viruses tested, occasionally against 2 of the 3 different viruses simultaneously. The occurrence in MS of virus-specific IgM may be a reflection of viral reactivation and/or polyclonal B cell activation.