Data collected over the past 6 years suggest that Coxsackie B viruses (CBV) play an important role in myalgic encephalomyelitis (ME). Since psychological upset is a feature of this illness, 247 patients, recently admitted to a psychiatric hospital, were tested for neutralizing antibodies to CBV. A total of 12.5% had significantly raised CBV titres compared with 4-5% of 'well' control groups; the percentage positive was greatest (21%) in those aged 30-39 years. During 1985 and 1986 sera from 290 adults with ME were tested using the newly developed CBV IgM ELISA test; 37% were CBV IgM positive compared with 9% of 500 'well' adult controls. Forty-seven children, with ME were similarly tested during this period; 38% were positive, implying recent or active CBV infection. The combined use of this ELISA test and the virus probe techniques now available should further help to elucidate the exact role of CBV in this disabling illness.