The present study tested the hypothesis that murine (m)IFN- or mIFN-₂ can eliminate cardiac viral load and protect cardiomyocytes from injury in animals infected with CVB3. CVB3-inoculated male Balb/c mice exhibited signs of illness, including lethargy, progressive weight loss and death (10% on day 3 and 100% on day 8). Cardiac viral load was high (4277 ± 1009 PFU and 25 ± 5 copies CVB3/HPRT1 mRNA) on day 4. The cardiac tissue exhibited severe inflammatory infiltration and myocyte damage with an average myocarditis index pathology score of 2.1 ± 0.2 on day 7. Most of the mice infected with CVB3 also developed epicarditis and 55% had intraventricular thrombi present. Treatment with mIFN- (2.5 to 10 MIU/kg) dose-dependently improved the general health status in CVB3-inoculated mice, as evidenced by reduction in weight loss, prevention of death, elimination of cardiac viral load, protection of myocytes from injury, descrease in inflammatory cell infiltration, and attenuation of intraventricular thrombus formation. Treatment with 10 MIU/kg mIFN-₂ resulted in a similar level of efficacy to that induced by 5 MIU/kg mIFN-, with the exception that mIFN-2 did not reduce cardiac CVB3 mRNA. However, mIFN-₂ , but not any dose group of mIFN-, significantly attenuated CVB3-induced epicarditis. These data demonstrated antiviral effects for both mIFN- and mIFN-2, which lead to protection of the mice from CVB3-induced myocarditis. However, the potential mechanisms leading to differential host response to the two isoforms of mIFN remain to be elucidated.