Advances in molecular biology have brought more refined techniques to the study of the structure and function of HLA molecules and the biology of arthritogenic bacteria. Site-directed mutations in HLA and bacterial genes have focused attention on amino acids that are playing a critical role in the dynamic interaction of host and pathogen. The role of HLA-B27 in conferring disease susceptibility is being evaluated in both human disease and in B27 transgenic animal models. The continued interchange between clinical research in the spondyloarthropathies and the in vitro systems analyzing the immunology and microbiology of these diseases has maintained a high level of interest in this field of arthritis research.