A dipeptide antigen determinant is disclosed where haptenic Cit and Cys residues are connected by means of a peptide bond having the formula III:where A is —NH2, B is —O—, D is —NH—, F is —NHCO—, or —NR—, where R is hydrogen or acyl, G is —CONH—, or —COOR, wherein R is alkyl, E is (CH2)n′, wherein n′ is an integer from 1 to 6, S is —SH, or —SS—, on the condition that free thiol (SH) of Cys is crosslinked by a disulfide bond to another Cys. The antigen determinant reacts sensitively and specifically with serum autoantibodies present in patients suffering from rheumatoid arthritis (RA) which is useful for diagnosing or treating RA.
