Precipitating antibodies occurring in 26 "pure" polymyositis (PM) patients and 22 "pure" dermatomyositis (DM) patients were studied by double immunodiffusion using calf thymus nuclear extract and whole calf thymus extract. Seventeen of 26 (65%) PM sera and 13 of 22 (59%) DM sera showed positive reactions, and 18, 10, and 2 sera exhibited 1, 2, and 3 precipitin lines, respectively. Common specificities were established by immunodiffusion for the 11 PM and 6 DM sera with strong precipitin lines. These were classed into three groups of shared specificity and six reactions specific for individual sera. These 9 calf thymus antigens were shown to be different from the known soluble tissue antigens Sm, nuclear RNP, Ro, and La. A 600-fold purification of the most frequently appearing system designated Jo-1 was achieved and its clinical specificity for patients with PM syndromes was established.