76 patients with the postviral fatigue syndrome (PVFS) and 30 matched controls were investigated. Virus isolation was attempted from concentrated faecal samples by direct culture and after acid dissociation of virus from antibody. Positive cultures of enteroviruses were obtained from 17 (22%) patients and 2 (7%) controls. An enterovirus-group-specific monoclonal antibody, 5-D8/1, directed against the VP1 polypeptide, was used to detect enteroviral antigen in the circulation, either free or complexed with antibody. VP1 antigen was detected in the serum of 44 (51%) of a further group of 87 PVFS patients. The number of patients positive for VP1 antigen was greater (42/44) when IgM complexes were detectable than when they were not (2/23). 1 year later, the 17 patients of the first group of 76 with positive cultures were again studied. The same virus was again isolated from 5 (29%), 13 (76%) had detectable IgM responses to enteroviruses, and 9 (53%) were positive for VP1 antigen in the serum. These results show that chronic infection with enteroviruses occurs in many PVFS patients and that detection of enterovirus antigen in the serum is a sensitive and satisfactory method for investigating infection in these patients.