1. Twenty-eight patients with active definite primary ankylosing spondylitis and fifty-four healthy control subjects were studied. 2. The HLA-B27 antigen was found in 75% of patients and 3.7% of controls. 3. Fecal samples from these subjects were cultured for gram-negative enteric bacteria on two occasions within one month. Positive cultures for Klebsiella sp were found in 32.1% of patients and in 22.2% of healthy controls, but this difference was not statistically significant. All other microorganisms detected were qualitatively and quantitatively similar in both groups. 4. Significantly increased mean values of serum IgA levels were found in the patient group when compared with the control group (P less than 0.01). The mean serum IgG and IgM levels did not differ statistically between the two groups. There was no correlation between any laboratory or clinical parameter and presence of Klebsiella sp carriage in ankylosing spondylitis patients. 5. These data are consistent with the view that a long time elapses between exposure to a trigger factor and clinical manifestations of the disease.