Serum C reactive protein (CRP) and erythrocyte sedimentation rate (ESR) were measured on 469 occasions in 149 patients with ankylosing spondylitis who had also been investigated for faecal carriage of klebsiella/enterobacter species on the same occasions. Raised values of CRP and ESR correlated with each other (P less than 0.001) as well as with clinically assessed active disease episodes (P less than 0.001). Patients with positive cultures for klebsiella were found to have higher mean values for CRP and ESR than those with negative cultures (P less than 0.025). CRP appears to be a useful marker of disease activity in ankylosing spondylitis.