The efficacy of the synthetic nucleoside ribavirin in the therapy of experimental myocarditis caused by coxsackievirus B3 and its effects on peripheral and splenic lymphocyte subsets were investigated. Two week old male C3H/He mice were inoculated with coxsackievirus B3. Ribavirin was administered subcutaneously on days 0 to 15 (experiment I) and on days 4 to 15 (experiment II) in a dose of 200 (experiment I: Group 2, n = 10; experiment II: Group 5, n = 10) or 400 mg/kg per day (experiment I: Group 3, n = 10; experiment II: Group 6, n = 10). Control mice (experiment I: Group 1, n = 15; experiment II: Group 4, n = 14) received an injection of saline solution. In experiment I, mice treated with ribavirin survived significantly longer than did control mice (p = 0.005, Group 1 versus Group 2; p = 0.001, Group 1 versus Group 3). In experiment II, the survival rate on day 15 in ribavirin-treated mice was high compared with that in control mice (Group 4 = 14.3%, Group 5 = 20%, Group 6 = 50%). Myocardial viral titers on days 5 to 8 were significantly lower in ribavirin-treated mice of both experiments than in control mice.(ABSTRACT TRUNCATED AT 250 WORDS)