Coxsackieviruses BI-B4 were inoculated intraperitoneally into 48-hr-old, 14-day-old, and three- to five-month-old Swiss-Webster mice. Immediate death occurred only among mice <48 hr old, which died from fulminant encephalitis. Older mice usually survived. Myocarditis ensued in mice <48 hr old that were infected with coxsackieviruses Bland B4. Several of the surviving mice developed left ventricular aneurysms, which resulted from transmural necrotizing myocarditis. In this group (coxsackieviruses Bl and B4), pathologic changes in the heart were synchronous with maximal cardiac titers of virus. Fourteen-day-old mice infected with coxsackieviruses B2 and B3 developed nontransmural necrotizing myocarditis in which maximal pathologic changes followed peak cardiac titers of virus by several days, whereas three- to five-month-old mice infected with coxsackieviruses Bl, B2, B3, or B4 showed maximal susceptibility to destructive lesions in the exocrine glandular pancreas. Therefore, specific susceptibilities to infection with coxsackieviruses group B vary with age of the mouse, virus type (and strain), and organ.