Jo-1 antigen was purified from the rabbit thymus acetone powder. Using it as antigen, we found anti-Jo-1 antibody in 25% of polymyositis, 7.1% in dermatomyositis, none in other connective tissue diseases nor non-connective tissue diseases. Other antinuclear antibodies, eg anti-RNP, anti-SSA were also found in our polymyositis-dermatomyositis patients. Our study suggested, the positivity of anti-Jo 1 antibody not only confirmed the patient's diagnosis of polymyositis but also demonstrated the autoimmune pathogenesis of this disease.