Antibodies to Salmonellae, Yersiniae, Campylobacter jejuni, Borrelia burgdorferi, Klebsiella pneumoniae, Escherichia coli, Proteus mirabilis and Chlamydia trachomatis were measured by ELISA in the sera of 99 patients with ankylosing spondylitis. Increased prevalence of IgA and IgG class antibodies against K. pneumoniae and of IgA class against E. coli was observed in ankylosing spondylitis. No clear correlation between the disease activity and occurrence of antibodies was revealed. The results are in line with the previously published findings suggesting that K. pneumoniae may have a role in the aetiopathogenesis of ankylosing spondylitis.