Sera from rabbits immunized with HLA-B27 lymphocytes showed increased activity against klebsiellal enterobacter antigens using immunodiffusion, bacterial agglutination (P less than 0.025), haemagglutination (P less than 0.001) and radiobinding assays (P less than 0.001). Immunoprecipitin lines were also produced by these antilymphocyte sera against extracts from Yersinia enterocolitica and Shigella sonnci microorganisms. Rabbit anti-klebsiella sera had lymphocytotoxic activity against HLA-B27 lymphocytes obtained from patients with ankylosing spondylitis (P less than 0.001). These results suggest partial cross-reactivity between some antigens found in several Gram-negative microorganisms and HLA-B27 lymphocytes.