Anti-HLA-B27 monoclonal antibody M2, which was relatively specific for human histocompatibility antigen HLA-B27, was used to test several bacteria, some of which could potentially induce chronic arthritis in HLA-B27-positive individuals. Using the Western blot procedure, we observed positive reactions with 80,000- and 60,000-dalton antigens with one strain of Klebsiella pneumoniae. Reactivity was not observed with five other monoclonal antibodies which were not reactive with HLA-B27 antigens, nor was reactivity observed with seven other gram-negative bacteria, irrespective of their arthritis-causing potential. To test the validity of our observation, the 80,000-dalton Klebsiella cross-reactive antigen was isolated and used to generate an immune guinea pig serum. We found that the reactivity of this guinea pig serum with Klebsiella envelopes in an enzyme-linked immunosorbent assay was adversely affected by absorption with HLA-B27-positive cells. Our results support the existence of mimicry between HLA-B27 antigens and bacteria.