The binding ability of 23 overlapping peptides, all derived from the CB11 fragment of CII, was tested on several HLA-DR molecules associated or not with disease susceptibility. These experiments were performed on a variety of cells expressing different HLA-DR molecules, using both indirect and direct binding assays. The CII (256-271) fragment was shown to bind to a restricted population among which the HLA-DR molecules associated with susceptibility to rheumatoid arthritis. The results also clearly indicate that the binding specificity of CII (256-271), among the DR4 molecules, is controlled by the nature of the HLA-DR molecule beta-chain residues 71 and 74, residues previously shown by X-ray crystallography to be involved in the HLA-DR/peptide interaction. The human CII (256-271) peptide is thus likely to play a role in the disease process.