A monoclonal antibody, Ye-1, was generated by immunizing BALB/c mice with Yersinia enterocolitica. This antibody also reacted with all of 12 B27 positive lymphoblastoid cell lines, but only four of 31 B27 negative ones. Three of the four reactive B27 negative cell lines were B7 positive. A B27 positive cell line which has lost the B27 expression because of experimentally-induced mutation became unreactive with the Ye-1. These findings support the possibility that there is cross-reactivity between HLA-B27 antigens and Y. enterocolitica.