To evaluate the antigenicity of a peptide representing a part of the second extracellular loop of the human muscarinic acetylcholine receptor 3 (m3AChR) with autoimmune sera from primary Sjogren's syndrome (pSS), enzyme-linked immunosorbent assays (ELISAs) were developed. On the basis of the computer-predicted data, a 16-mer synthetic peptide KRTVPPGECFIQFLSE (KRSE213-228) was produced by solid-phase peptide synthesis. cDNA coding for the KRSE peptide was chemically synthetized and utilized to express the recombinant glutathione S-transferase (GST)-KRSE fusion protein. The immunoreactivities of the two antigens were tested in ELISAs with the sera of 40 pSS patients and 40 healthy controls. The specificity of the reaction was confirmed by inhibition assays and immunoblottings. The pSS sera resulted in significantly higher mean optical densities than those of the healthy controls (KRSE: 0.4149 vs 0.1494, p<0.0001; GST-KRSE 0.4765 vs 0.1764, p<0.0001). The immunological recognition with the recombinant fusion antigen was significantly better than that for the free peptide (p=0.0068). The sensitivities of the assays were 77.5% (KRSE) and 97% (GST-KRSE). The results of the concentration-dependent inhibition assays by the two systems of peptide presentation indicated that the KRSE sequence is specific for pSS sera. This is the first demonstration of the antigenicity of a novel peptide fragment of the human m3AChR in pSS. The analysed peptide could be of diagnostic relevance.