The phenotype HLA-B27 is common in patients who develop reactive arthritis after having an infection. One hypothesis concerning the pathogenesis of reactive arthritis is that molecular mimicry between HLA-B27 and certain bacterial components might be involved. It is known that an infection withYersiniais commonly associated with reactive arthritis in B27 positive patients. Therefore, we were interested to investigate whether cross-reactivity betweenYersiniaand HLA-B27 exists. A gene library ofYersinia pseudotuberculosiswas created in the plasmid vector pUC13. One of the resulting clones contained a gene encoding an intracytoplasmic protein that seems to have partial epitope identity with HLA-B27. It reacted in western blot. ELISA and immunoprecipitation with three different HLA-B27 specific monoclonal and polyclonal antibodies of the IgG and IgM class. However DNA-sequencing of the clonedYersiniagene and the predicted amino acid sequence revealed only a very remote similarity with HLA-B27 in the primary structure. Instead, an extremely high degree of similarity with the ribosomal protein L4 of the S10 operon ofEscherichia coliwas identified indicating that the protein encoded by the clonedY. pseudotuberculosisgene is a corresponding ribosomal protein.