Cross-reactivity between antibodies to 2 strains of Klebsiella pneumoniae (K43 and F77) and the peripheral blood lymphocytes of patients with ankylosing spondylitis (AS) was examined in 3 separate antibody binding and cytotoxicity assays. Using K pneumoniae antisera in a chromium release cytotoxicity assay, we found no difference in the reactions of cells from AS patients and those from control subjects. This result contrasts with the results of previous studies. Similarly, using an enzyme-linked immunosorbent assay, we detected no significant increase in antibody binding to peripheral blood mononuclear cells (PBMC) in HLA-B27 positive patients with AS. Low levels of antibody binding were detected by a fluoresceinated antibody binding assay; however, normal rabbit serum, which was used as a control, was shown to have a binding affinity for PBMC that was significantly greater than that of specific K pneumoniae antisera. The results of our present study do not support the concept of a specific cross-reactivity between antibodies to K pneumoniae and the PBMC of patients with AS who are HLA-B27 positive.