BACKGROUND Antimicrob drugs and immune system interaction has been studied since the pioneer works of Metchnikoff. After the introduction of antibiotics in clinical practice this area has attracted little attention of investigators, because of the lack of standards. This is the reason that the studying of the influence of antibiotics on immune system is still at its beginning. OBJECTIVE To point out the immunomodulatory action of some antibiotics on certain components of immune system. RESULTS The literature findings show that antibiotics express immunomodulatory action on some components of immune system such as fagocytes (polymorphonucleary, macrophages, monocytes), cytokines, immunoglobulines, and on cellular immunity. The principles of antibiotics action on phagocyte are the inhibition of chemotaxis and oxidants production. Macrolides applied for a short time enhance the phagocytic functions while their long use leads to immunosupression. Some cephalosporines and rifampicin in therapeutic doses inhibit the oxydative metabolism, of macrophages. Tetracyclines, clindamycines, chloramphenicol and tobramycin inhibit the synthesis of superoxyd anione. The action of some antibiotics on cytokine and specific antibodies is also important. Cellular immunity can be affected as well. After administration of certain antibiotics it takes 1-2 weeks to reestablish normal cellular immunity, and for other even more. CONCLUSIONS There is still no clear standing on real effects of antibiotics on the immune system. Clinicians should search for more information from this new-old field of investigation in order to give more adequate therapy to patients.