Purified chick type II collagen was cleaved with cyanogen bromide (CB), and the resulting peptides isolated and renatured. Sera from arthritic DBA/1 mice, immunized with chick type II collagen, were tested for reactivity with each peptide. The sera preferentially recognized peptides 11, 10, and 8, in that order. Some reactivity was also detected to peptides 9, 7, and 12. Because arthritis depends upon binding of antibody to autologous type II collagen in the joint, sera were also tested for reactivity with mouse type II collagen. There was a strong positive correlation between reactivity with peptide 11 and reactivity with mouse collagen, but no correlation was found with any of the other peptides. Peptides 11, 10, and 8 were also used for immunization. Antibodies were detected in response to each of these peptides, but arthritis developed only in mice immunized with peptide 11. We conclude that a major immunogenic and arthritogenic epitope on type II collagen resides in the region of the molecule represented by CB peptide 11.