Inoculation of Coxsackie B1 virus (CB1) in newborn CD1 Swiss mice induces a chronic myositis of proximal hindlimb muscles (CB1 myositis). To study the possible role of the virus dose, and of antiviral antibodies in the development of CB1 myositis, we infected groups of newborn mice with six CB1 doses, ranging from 30 to 10,000 plaque forming units (pfu); after 4 and 8 weeks we determined morbidity and antiviral antibody titer, and quantified histopathological changes. At 4 weeks, morbidity and mononuclear cell infiltration differed significantly for the various groups, with the most prominent changes in 300 pfu animals. At 4 and 8 weeks diseased animals had significantly higher antibody titers than clinically unaffected animals, and at 4 weeks myopathic and infiltrative changes correlated positively with the serum antibody titer. Our data indicate that the virus dose plays a pathogenic role in CB1 myositis, and they suggest further study on the role of humoral immune mechanisms in the early phase of CB1 myositis.