The persistence of coxsackievirus B1 in the muscles of mice with coxsackievirus B1-induced chronic myositis was investigated. Neonatal CD1 Swiss mice were inoculated with a myositis-causing variant of coxsackievirus B1 (Tucson strain). Hamstring muscle samples of diseased mice obtained at various times after inoculation were examined for the presence of infectious virus, viral RNA and histological abnormalities. Viral RNA was detected up to 4 weeks after initiation of infection, whereas virus could be isolated from hamstring muscles for up to 2 weeks. Thereafter no sign of infection was demonstrated although histological abnormalities remained present for the entire observation period of 16 weeks. That viral RNA was detectable for only 2 weeks after tissues became negative for infectious virus suggests that the infection slowly waned rather than the viral RNA persisting. Hence, it is concluded that coxsackievirus B1 plays an essential role in the initiation of myositis but not in the maintenance of the chronic phase.