OBJECTIVE To identify the 68-kd target of antibody in serum samples from patients with idiopathic, progressive, bilateral sensorineural hearing loss. METHODS To purify target protein from renal extracts using gel filtration, ion-exchange chromatography, and polyacrylamide gel electrophoresis and to transfer to nitrocellulose membranes. The purified protein was digested with trypsin, and peptide fragments were separated by high-pressure liquid chromatography. RESULTS One fraction obtained by high-performance liquid chromatography contained a peptide of 2776 molecular weight. The sequence of a stretch of 22 amino acids within this peptide was identical to that of amino acids 424 through 445 of heat shock protein 70 (HSP70). On Western blotting, monoclonal antibody directed against HSP70 (but capable of recognizing both constitutive HSP70 [HSC70] and stress-inducible HSP70) reacted with the purified 68-kd protein. We compared the reactivity of serum samples from six patients with idiopathic, progressive, bilateral sensorineural hearing loss, as well as monoclonal antibody to HSC70, and monoclonal antibody to HSP70 with renal extract. The pattern obtained suggested that patient antibodies are preferentially directed at HSP70. CONCLUSIONS The target of antibody in serum samples from patients with idiopathic, progressive, bilateral sensorineural hearing loss is HSP70.