Chick type II collagen (CII), a protein commonly found in joint cartilage, induces an autoimmune arthritis when administered to susceptible strains of mice. A cyanogen bromide fragment of CII, CB11, contains the requisite epitopes critical for inducing collagen-induced arthritis. If administered as a tolerogen, however, before immunization, CB11 prevents the onset of disease. Therefore, delineation of structural elements of CB11 that can regulate autoreactive T cells became the goal of this study. To delineate the structural elements of CB11 antigenic to T cells, 14 peptides containing overlapping sequences of CB11 were generated. Mononuclear cells from CII-immunized DBA/1 mice were cultured with these peptides and the resulting supernatants examined for the production of IFN-gamma. Two peptides, CII 181-209 and CII 245-270, generated the greatest responses. The ability of these two peptides to regulate arthritis was tested by administering them to neonatal DBA/1 mice as tolerogens before immunization with CII. Both peptides suppressed the incidence of arthritis whereas no other peptide used as a tolerogen significantly altered the course of the disease. T cells from four arthritis-resistant murine strains did not recognize either peptide when immunized with CII, whereas cells from the disease-susceptible B10.Q mice responded well to both. Thus, the coincidence of T cell responses to CII 181-209 and CII 245-270 in CIA-susceptible mice and the lack of response in disease-resistant strains or CII-tolerized mice identify these two peptides as containing important T cell epitopes that regulate CIA.