One-hundred and twenty-four patients with spondylarthropathies were studied for antibodies to the peptides from HLA-B27.1 and Klebsiella pneumoniae nitrogenase which share a QTDRED hexamer sequence. Of 60 male Norwegian ankylosing spondylitis (AS) patients 23.3% showed positive ELISA reactivity for B27.1 peptide compared with 4% of Norwegian male controls (P less than 0.10). This difference was not observed among patients and controls from New Mexico. All patients with anti-B27.1 antibody were HLA-B27+. Antibody to B27.1 peptide was present in 20% of normal female controls with at least one previous pregnancy. No female control without previous pregnancy showed positive anti-B27.1 peptide reactivity. Anti-Klebsiella peptide antibody was neither significantly elevated in AS nor correlated with anti-B27.1 peptide antibody. Significant migration inhibition by these peptides was not observed in AS or normal controls. The possible influence of epitope conformation, rather than sequence homology, in potentially cross-reacting determinants shared by bacterial antigens and human Class I molecules requires further study.