Autoimmune diseases are characterized serologically by the presence of antibodies to specific autoantigens. Antibodies to the two antigens Ro/SSA and La/SSB are found in patients with primary (pSS) and secondary Sjögren's syndrome (sSS). To explore if differences in the fine specificity of these autoantibodies could be distinguished in sera from patients with primary (n = 17) and secondary (n = 20) Sjögren's syndrome, sera were analysed by immunoblotting and ELISA using recombinant antigens and synthetic peptides. Minor differences were detected when the frequencies of the Ro 60 kD, Ro 52 kD and La autoantibody specificities to full-length proteins in the pSS and sSS groups were compared. However, when reactivity to different parts of the Ro 60 kD antigen was analysed, including recombinant fragments encompassing amino acid (aa) 1-134, aa 181-320 and aa 397-525, only two sera, both from pSS patients, reacted to the aminoterminal fragment aa 1-134, and 3/4 sera that reacted with the carboxyterminal aa 397-525 fragment derived from sSS patients. Of all the anti-Ro 60 kD positive sera, 80% reacted with the middle fragment encompassing aa residues 181-320. The fine specificity of the autoantibodies reacting with this 181-320 aa region was further mapped with synthetic peptides, and a peptide (VSLVCEKLCNEKLLKKARIH) recognized by 8 out of 16 sera from both pSS and sSS patients was identified.