In order to address the question whether the virus or immune reactions to the heart induce cardiac damage in the murine model of coxsackievirus B3 induced myocarditis, calcium-resistant cardiac myocytes of a myocarditis susceptible strain of mice were isolated and exposed to myocarditic coxsackievirus B3. The tetrazolium salt MTT was used to visualize the effect of the virus upon cell viability by inverse light microscopy and by an EA test. Coxsackievirus B3 infected isolated adult cardiac myocytes as well as infected cultured cardiac fibroblasts were examined. In vitro the virus killed the myocytes within 16 h of infection whereas the fibroblasts survived the infection. Since coxsackievirus B3 is able to kill cardiac myocytes by itself, immunosuppressive treatment of acute coxsackievirus B3 induced myocarditis may be harmful by eliminating host immune defence mechanisms and, therefore, may lead to an enhanced viral spread and virus induced myocyte necrosis in the heart.