Quinolones have a broad antibacterial spectrum against Gram-negative and Gram-positive bacteria. The compounds, however, have a few adverse effects, such as convulsion and toxicity to articular cartilage. We observed that some quinolones such as Naldixic acid, Ofloxacin, and Norfloxacin have osteoclast-inducing effects. All quinolones we tested produced tumor necrosis factor-alpha and prostaglandin E₂, and have potency as osteoclast inducers to cultured cells. These results suggest that some quinolones affect osteoclast induction or activation, and this may be related to the production of tumor necrosis factor-alpha (TNF-alpha) and prostaglandin E₂ (PGE₂).