Muscle biopsy specimens of a patient with polymyositis showed crystalline structures resembling picornavirus aggregates within muscle cells. The patient's serum reacted in an indirect immunofluorescence assay with autologous muscle cells. A strongly positive immunofluorescence staining was also noted when a section of muscle tissue was reacted with coxsackievirus A9 antiserum, and a weakly positive reaction was noted with coxsackievirus B2 antiserum. No staining was observed after treatment with antiserum to poliovirus type 1 or echovirus types 11 and 22.