A search for the presence of Klebsiella-Enterobacter spp. and Yersinia enterocolitica in urine and faeces of 63 patients with ankylosing spondylitis was conducted because these microorganisms have been demonstrated to cross-react immunologically with HL-A B27 positive lymphocytes. The patients were graded into three groups on the basis of disease activity. Klebsiella spp. were found in the faeces of 13 (93%) of the 14 patients with ‘active’ disease, 10 (48%) of the 21 patients with ‘probably active’ disease and in one (4%) of the 28 patients with ‘inactive’ disease. Positive cultures were also obtained in 47 (38%) of 124 controls. It is suggested that the presence of Klebsiella spp. in faecal cultures may be associated with ‘active’ disease in patients with ankylosing spondylitis.