This study describes aspects of the specific changes which occur in the myocardial cells of Coxsackie B4 virus infected suckling mice. These changes consisted of cellular shedding, membrane-vesicle complex formation, disappearance of Golgi body, nuclear shrinkage and densification of chromatin and an increase in ribosomal population. These changes are in addition to the nonspecific changes which probably result as a nonspecific cellular response to injury. © 1969 Springer-Verlag.