The role of coxsackie B viruses (CBV) in myo/pericarditis has been well documented; however, interpretation of static high neutralising antibody titres in individual patients has always been difficult. In introducing the mu-antibody capture ELISA test for the detection of CBV-specific IgM, we hoped to overcome this problem. A regimen for the routine serological diagnosis of CBV infections was introduced, using the CBV IgM ELISA as a screening test, followed by neutralisation tests (NT) to confirm the positive results. Seven hundred and sixty patients and 304 healthy adult controls were tested. The percentage CBV IgM positive in each of the clinical categories myo/pericarditis (33%) chest pain (22%), myalgic encephalomyelitis (31%), myalgia/Bornholm (19%) and controls (9%) was similar to those found in previous studies using NT alone. Cross-reactions with other enteroviruses, including hepatitis A (Enterovirus 72), were observed but did not prove to be a problem in the illness studied, since most involved adults. Both homotypic and heterotypic CBV IgM responses were found. Matching IgM and NT indicated a recent CBV infection. Positive IgM with negative NT titres suggested a recent infection with an enterovirus other than a CBV.