The occurrence of certain antibacterial antibodies was studied in the sera of 22 healthy donors (HD) and 66 patients with different diseases. The cases investigated included 22 rheumatoid arthritis (RA), 22 non-arthritic-psoriasis (NAP), and 22 psoriatic arthritis (PA) patients. A complement fixation test was used with Yersinia enterocolitica 0:3 type (YEC), Yersinia pseudotuberculosis (YPT), Campylobacter jejuni (CJ), and Campylobacter fetus (CF) antigens; the detection of anti-Chlamydia trachomatis (CT) antibodies was carried out using an immunoperoxidase colorimetric slide test that allowed the detection of isotypes of specific antibodies. It was found that the synthesis of anti-CF, CJ, YEC, and YPT antibodies in NAP patients does not differ significantly from that of the HD group; on the contrary, the antibody levels were statistically higher in PA than in the other disease groups or in the healthy controls, although only anti-CF antibodies seemed to significantly differentiate (p = 0.000003) the PA group from the others. Anti-CT IgA antibody titers were found to be significantly higher in the PA as well as in the RA groups when compared with the controls, while the antibody levels in NAP patients showed no clear-cut difference with respect to those of either the arthritic patients or the healthy controls. By showing that anti-enterobacterial antibodies are increased in PA but not in NAP patients, our data furnish additional support to the thesis of a pathogenic role of bacterial infections in psoriatic arthritis.