Carditis developed 7 days after the administration of murine cytomegalovirus to neonatal, young adult or aged mice of varying sensitivity to lethal infection with this virus. The inflammation persisted for up to 80 days, but infected myocardial cells were rare and were not seen after day 10. The inflammatory cells comprised macrophages (up to 30%) and T cells (up to 80%), with a high ratio of Lyt2+ to L3T4+ cells throughout. Although the H-2 genotype affects murine cytomegalovirus replication at the level of individual cells, and hence resistance to lethal infection, it did not determine resistance to cardiopathy per se. However BALB/c, BALB.B, and BALB.K mice developed persistent myocarditis regardless of age at infection, and age-related cardiopathy was frequent and severe in infected and uninfected mice. B10 and B10.BR mice also developed myocarditis after neonatal infection, but inflammation resolved rapidly after adult infection and age-related cardiopathy was correspondingly mild. C3H mice exhibited minimal carditis after neonatal or adult infection. However neonatal infection appears to accelerate age-related cardiopathy, which is severe in retired breeders of this strain.