N:NIH(S) II nu/nu (athymic) and +/nu (euthymic) mice were inoculated with coxsackievirus B3 (CBV-3) and examined at various times after infection for virus titres in the heart, myocarditis and serum neutralizing antibodies. Virus was recovered from the hearts of nu/nu mice for up to 94 days post-inoculation, but was not recovered from the hearts of any +/nu mice beyond 14 days. Inflammatory cell infiltration and necrosis were present in the hearts of +/nu mice at all harvest times (7, 14, 21 and 28 days). Inflammation and necrosis did not become evident in nu/nu mice until 14 days post-inoculation, and was the present in mice from each harvest until the end of the experiment (94 days). In athymic mice, myocarditis showed a strong correlation with persistence of CBV-3 in the heart. In N:NIH(S) II mice, the presence (+/nu) or absence (nu/nu) of a thymus had a major influence on the clearance of virus from the heart and on the development of myocarditis.