The recognition that periodontal diseases are primarily caused by specific microorganisms has led researchers to explore the possibility that antibiotics may enhance the effect of mechanical debridement procedures such as scaling and surgery. For some selected periodontal diseases, this has proven to be true. This paper will review systemically-administered antibiotics and the clinical studies and case reports supporting their use. In periodontal therapy, the tetracyclines are the most commonly-used antibiotics in the United States. Tetracycline hydrochloride, minocycline, and doxycycline have been shown to inhibit in vitro most putative periodontal pathogens. Several studies support the use of tetracyclines in the treatment of localized juvenile periodontitis. Penicillins such as amoxicillin are effective in vitro against most periodontal pathogens but have limited efficacy due to the presence of beta-lactamases in gingival fluid. Amoxicillin/clavulanate potassium (Au) has proven effective in treating adult refractory periodontitis characterized by a Gram-positive flora. Metronidazole is an effective adjunct in adult periodontitis associated with high numbers of "black-pigmented Bacteroides" ad spirochetes. A combination of metronidazole and amoxicillin produces a synergistic effect against A. actinomycetemcomitans and has been shown to be effective at eliminating this organism. Clindamycin is an effective adjunct in the treatment of adult refractory periodontitis associated with a predominantly Gram-negative flora. The use of macrolides, quinolones, and combinations of antibiotics is discussed. Clinical studies do not support the use of systemically-administered antibiotics in routine adult periodontitis. Clinical studies do, however, support the use of antibiotics in the treatment of specific periodontal diseases.