In the course of studying antinuclear antibodies in the rheumatic diseases, a new precipitin reaction (provisionally referred to as PM-1) was observed between calf thymus nuclear extract and polymyositis sera. Objectives of this study were to further define the immunologic nature of this reaction and to determine its specificity for polymyositis. Immunodiffusion studies using calf thymus nuclear extract revealed the PM-1 precipitin line in 17 of 28 patients with polymyositis. This reaction was not produced by sera of 460 patients with other diseases. Enzyme and heat treatments of the nuclear extract showed that PM-1 was distinct from native DNA, ribonucleoprotein, and Sm antigens. Fractionation of PM-1-positive serum by 30% ammonium sulphate and Sephadex G-200 chromatography revealed that the factor producing the PM-1 precipitin reaction was in a serum fraction which showed only IgG by immunoelectrphoresis against anti-whole human serum. Because of the apparent strong specificity, the PM-1 system may represent a marker antibody for polymyositis.
