Susceptibility to collagen-induced arthritis (CIA), a murine model of autoimmune arthritis, is strongly linked to only two major histocompatibility complex (MHC) haplotypes, H-2q and H-2r. In order to identify the determinants of type II collagen (CII) required to induce arthritis in H-2r-bearing mice, B10.RIII mice were immunized with bovine, chick or human CII. Only bovine CII induced significant arthritis and autoantibodies. When the major CNBr peptides of bovine collagen were isolated and used for immunization, only mice immunized with CB8, representing CII 403-551, developed arthritis. To identify immunogenic epitope(s) within CB8, a panel of synthetic peptides representing overlapping sequences of the bovine peptide was generated. When each peptide was cultured with T cells from B10.RIII mice immunized with CII, one peptide, representing CII 430-466, contained a major T-cell epitope. By using an in vitro lymphokine production assay, the T-cell epitope was further narrowed to CII 442-456. These findings suggest that a T-cell determinant important for the initiation of arthritis in B10.RIII (H-2r) mice is located within a 15 amino acid sequence, residues 442-456 of bovine CII.