Enteric infections with Gram-negative bacteria are thought to play an important part in HLA-B27-associated disease such as Reiter's syndrome and reactive arthritis. But the role of bacterial infections in HLA-B27-positive ankylosing spondylitis (AS) and acute anterior uveitis (AU) is still controversial. A special interest has recently been devoted to the role of klebsiella infection in HLA-B27-associated disease. We studied the humoral immune response against a 'cross-reactive' strain of Klebsiella pneumoniae in 62 patients with anterior uveitis and 33 healthy controls. The anterior uveitis patients were subdivided into 25 HLA-B27-negative patients without AS (B27- AU+ AS-), 17 HLA-B27-positive patients without ankylosing spondylitis (B27+ AU+ AS-), and 19 HLA-B27-positive patients with ankylosing spondylitis (B27+ AU+ AS+). Total serum IgA was higher in patients than in controls in both the B27+ AU+ AS+ and B27+ AU+ AS- patients but not in the B27- AU+ AS- group. No abnormalities were observed in the total serum IgG levels. The level of both the IgG and IgA klebsiella antibodies did not differ in the various patient groups tested as compared with the controls. Comparisons between the patient groups showed that the IgG anti-klebsiella response was higher in B27-positive patients patients without AS than in those with AS. These results suggest that stimulation of mucosal surfaces may play a role in HLA-B27-associated anterior uveitis. Whether klebsiella organisms are involved in this stimulation remains unclear.