Anti-PL7 antibodies to threonyl-tRNA synthetase purified from beef liver were observed in the serum of a dermatopolymyositis patient. They were identified by CIE and DID using a standard anti-PL7 antiserum and quantified by ELISA in serum samples taken over a five year period. The level of antibodies against not only threonyl-tRNA synthetase but also several muscle proteins including tropomyosin, was strongly correlated with disease activity and treatment. This correlation may prove important both in monitoring the response of the patient to treatment and to the understanding of the aetiology of the disease.Autoantibodies to aminoacyl-tRNA synthetases are found in 30% of sera from patients with myositis, being highly specific for this disorder and strongly associated with complicating interstitial lung disease. So far, autoantibodies recognizing five different synthetases have been described: anti-Jo-1 (histidine), anti-PL-12 (alanine), anti-PL-7 (threonine), anti-EJ (glycine) and anti-OJ (isoleucine).