Relapsing polychondritis (RP) is a rare systemic autoimmune disease, characterized by bilateral auricular chondritis, polyarthritis, nasal chondritis, respiratory tract chondritis, and audiovestibular damage (14,22). Inflammation of external ears with tissue deformity leads to 'cauliflower' ears (14, 26). Inflammation of cartilage of the nasal septum sometimes results in a 'saddle-nose' appearance. Inflammation of tracheolaryngeal tracts produces obstruction of airways by collapse of tracheal rings and bronchi. Polyarthritis occurs in large and small joints in RP. However, the arthritis in RP is generally asymmetric, non-erosive, and sero-negative. These characteristics are different from those of rheumatoid arthritis (RA) (25). Based on the systemic inflammation of cartilage, autoimmunity to cartilagerelated components would be involved in the pathogenesis of RP. As cartilage-related autoantigens in RP, several proteins had been reported so far. They include type II collagen (5,8,23), type IX collagen (10), type XI collagen (35), and cartilage oligomeric matrix protein (13). Recently, matrillin-1, which is expressed mainly in the tracheal cartilage not in articular cartilage, Abstract: Relapsing polychondritis (RP) is a systemic inflammatory disease, in which autoimmunity to cartilage-related components is thought to be involved in its pathogenesis. However, the autoimmune profile in RP has not been studied fully. We therefore investigated autoantibodies/autoantigens in RP comprehensively, by 2-dimensional electrophoresis (2DE), subsequent western blotting (WB) and mass spectrometry, using cell-extracted proteins as the antigen source. As a result, we detected 15 autoantigens on 2DE-WB, and further identified five of them. On average, one RP serum recognized approximately 8 out of the 15 autoantigens. Frequencies of the autoantibodies to the 5 identified antigens of tubulin alpha ubiquitous/6, vimentin, alpha enolase, calreticulin, and colligin-1/-2 were 91%, 46%, 36%, 82%, and 36%, respectively. ELISA using recombinant proteins for them revealed that frequencies of the autoantibodies to tubulin alpha ubiquitous, vimentin, alpha enolase, calreticulin, and colligin-1 were 36%, 64%, 46%, 27%, and 18%, respectively. Our data demonstrated that the autoimmune reaction was not restricted to cartilagerelated components, rather a variety of autoimmune responses occurred in patients with RP, which may be involved in the pathophysiology of RP. In addition, the proteomic approach using cell-extracted proteins would be a powerful way to investigate autoantigens.