The Jo-1 antigen is histidyl-transfer ribonucleic acid (t-RNA) synthetase. This enzyme is partially responsible for attaching t-RNA to their cognate rRNA (2). The Jo-1 antigen migrates as a 53 kD protein on SDS-PAGE. Anti-Jo-1 autoantibodies were originally described as precipitating autoantibodies in sera of patients with polymyositis. It was later realized that the anti-Jo-1 antibodies were specific for patients with polymyositis. The target for the anti-Jo-1 antibodies was one of a family of distinct cellular enzymes, the aminoacyl t-RNA synthetases (1). The presence of autoantibodies against the Jo-1 antigen has been reported in up to 23% of polymyositis patients by immunodiffusion (3). Anti-Jo-1 antibodies are almost completely specific for myositis, being more common in polymyositis than dermato-myositis(4), and rare in children (5). The presence of anti-Jo-1 antibodies defines a distinct group of polymyositis patients with interstitial disease, arthritis, and fevers (6). The anti-Jo-1 response appears to be self-antigen driven, having a broad spectrotype over time undergoing isotype switching. Anti-Jo-1 antibodies also inhibit the function of human histidyl tRNA synthetase more than they do from other species (6).