C1R cells were used as they have no surface HLA-A/B expression, express large amounts of HLA-B*27, and have previously been used in a number of experiments to explore the contribution of HLA-B*27 to S. Typhimurium- and S. Enteritidis-induced reactive arthritis (32, 33, 39, 40). This evidence concerns the gene HLA-B and reactive arthritis.