HLA-B27 transgenic animal models suggest a role for CD4+ T lymphocytes in the pathogenesis of the spondyloarthropathies, and murine studies have raised the possibility that unusual forms of B27 may be involved in disease. We demonstrate that CD4+ T cells capable of recognizing B27 can be isolated from humans by coculture with the MHC class II-negative cell line T2 transfected with B27. These CD4+ T cells recognize a panel of B27-transfected cell lines that are defective in Ag-processing pathways, but not the nontransfected parental cell lines, in a CD4-dependent fashion. Inhibition of responses by the MHC class I-specific mAb w6/32 and the B27 binding mAb ME1 implicates the recognition of a form of B27 recognized by both of these Abs. We suggest that B27-reactive CD4+ T cells may be pathogenic in spondyloarthropathies, particularly if factors such as infection influence expression of abnormal forms of B27.