Clinical and in vitro studies have demonstrated that fluoroquinolones are toxic to chondrocytes; however, the exact mechanism of fluoroquinolone arthropathy is unknown. We investigated the toxicity of ciprofloxacin on normal cartilage and on cartilaginous tumors. Normal human cartilage, enchondroma, and chondrosarcoma explants were cultured either alone or with the addition of ciprofloxacin at 1, 10, or 20 mg/L of medium. Samples were collected up to twenty-one days after treatment and were processed for electron microscopy and conventional light microscopy. The specimens were characterized morphologically with use of conventional light microscopy, electron microscopy, and immunohistochemistry to identify extracellular matrix, cell proliferation, and apoptosis. Cultures of normal chondrocytes expressed type-II collagen. Electron microscopy revealed a large amount of glycogen in the cells; the presence of fat droplets, rough endoplasmic reticulum, and prominent Golgi apparatus; and a proteoglycan layer surrounding the cells. With prolonged ciprofloxacin treatment and with increased doses, there was an increase in dilated rough endoplasmic reticulum, the appearance of phagosomes, and disintegrated bundles of vimentin filaments. The treated chondrocytes showed a decrease in cell proliferation, but there was no induction of apoptosis or effect on the expression of extracellular matrix proteins. Ciprofloxacin-treated chondrosarcoma cultures and tissue samples showed changes in cartilage matrix composition. Ultrastructural analysis demonstrated clumped glycogen, dilation of endoplasmic reticulum, numerous abnormal lysosomes containing degeneration products, and a decreased proteoglycan deposit surrounding the tumor cells. Treated chondrosarcoma cells and tissue specimens did not proliferate, and apoptosis was induced. In contrast, the in vitro growth of other noncartilaginous malignant tumors like osteosarcoma and liposarcoma was unaffected by ciprofloxacin. Our results indicate that ciprofloxacin is toxic to chondrocytes. In vitro and in vivo treated chondrosarcomas are the most affected.