Polymorphonuclear leucocyte (PMN) functions (migration in vitro, chemiluminescence, O-2 production, and aggregation) were studied in 32 patients with previous yersinia arthritis (YA). PMNs of 11 HLA-B27 positive patients who had chronic or recurrent inflammatory symptoms showed O-2 production significantly higher than that of PMNs of 11 HLA-B27 positive patients without late manifestations. Also, PMNs of both HLA-B27 positive and negative patients tended to show chemotactic and chemokinetic migration rates higher than those of control cells of healthy HLA-B27 negative subjects. These functional aberrations may play a part in the development of the patients' inflammatory symptoms.