Autoantibodies directed against cellular antigens are frequently found in the connective tissue diseases (1,2). In myositis, an inflammatory condition of muscle, the antigens are often cytoplasmic rather than nuclear components (3, 4). We recently identified two such antigens as aminoacyl-tRNA synthetases; anti-Jo-1 antibody, found in 25-30% of myositis sera (2, 4), reacts with histidyl-tRNA synthetase (3), and PL-7 antibody, found in 5% of cases, is directed against threonyltRNA synthetase (5). Both antibodies immunoprecipitate ribonucleoprotein complexes containing the enzyme and its transfer RNA; they recognize the protein free of RNA, but do not recognize the RNA alone. Here we describe a third myositis-related autoantibody, PL-12, that inhibits alanyl-tRNA synthetase. Both the enzyme and tRNA Area are immunoprecipitated, but in this case, distinct sets of antibodies react with each component independently. Furthermore, antibody against the synthetase fails to recognize the enzyme bound to its cognate tRNA. We discuss the possibility that one antibody may be an antiidiotype to the other.