Using epitope scanning of 272 short, synthetic peptides representing the amino acid sequence of the CB-11 peptide of type II collagen, we have shown that five strains of rat, immunized with type II collagen, produce antibodies to a region 37-45 amino acids from the amino end of CB-11 peptide. Antibodies to this region always gave the highest binding values suggesting that it is an immunodominant region. Wistar rats immunized with a synthetic peptide representing this region, coupled to keyhole limpet haemocyanin, produced antibodies to this peptide which could still be detected at 1:4000 to 1:8000 dilution but none developed clinical arthritis. All sera also showed binding of antibodies to denatured bovine type II collagen but not to native type II collagen, keyhole limpet haemocyanin or to bovine serum albumin by ELISA. Sera from peptide-immunized rats were examined for antibody binding to the 272 short peptides of the CB-11 peptide and to the synthetic peptides representing shortened forms of the immunodominant region and forms of it with substituted amino acids. These results showed that the antibodies in the peptide-immunized rats were not identical to those produced to that peptide by rats immunized with type II collagen but may represent subpopulations of them. These findings suggest caution in interpreting the role of antibodies to individual peptides in arthritis induction without knowledge of their fine specificity.