Random peptide libraries (RPLs) screening with IDDM sera has identified 5 disease-specific 'mimotopes' displayed on phage (phagotopes). We characterised one phagotope (CH1p), by raising a rabbit antibody against the peptide insert on phage, which was employed in immunohistochemistry, Western blotting and cDNA libraries screening. The CH1p mimotope was detected in somatostatin cells of human islets and experimentally raised anti-osteopontin antibodies or human sera positive for the phagotope, detected a similar subpopulation of islet cells. The screening of cDNA library identified a clone corresponding to human osteopontin. In summary, RPLs proved to be successful in the identification of a novel islet-related autoantigen (osteopontin), whose significance in disease remains to be established.