Phage preparations used for phage therapy may have not only direct antibacterial action but also immunomodulating effects mediated by phages themselves as well as by bacterial antigens. Therefore phage application in patients with immune disorders, and especially with autoimmune diseases, requires special attention. The aim of this study was to investigate the effect of phage lysates (staphylococcal phages A3/R, phi200, and MS-1 cocktail, enterococcal phage 15/P, Pseudomonas phage 119x, and E. coli T4 phage) as well as purified T4 phage on the course of murine collagen-induced arthritis (CIA), commonly used as an animal model of rheumatoid arthritis. Intraperitoneal application of phage lysates or purified T4 phage did not aggravate the course of autoimmune joint disease. Moreover, although endotoxins are known to potentiate CIA, the systemic administration of phage lysate of Pseudomonas aeruginosa, which contains debris of this Gram-negative bacillus, did not significantly influence CIA although the sonicate of the corresponding bacterial strain did. Interestingly, a purified T4 phage revealed some anti-inflammatory activity when applied under the therapeutic scheme. Our preliminary results do not suggest that phages may aggravate the symptoms of rheumatoid arthritis. In contrast T4 phage may even exert an immunosuppressive effect.