Antimicrobial therapy with fluoroquinolones can be associated with tendinitis and other tendon disorders as an adverse reaction associated with this class of antimicrobials. Here we investigated aspects of the mechanism of quinolone-induced tendotoxicity in human tenocytes focussing mainly on the question whether fluoroquinolones may induce apoptosis. Monolayers of human tenocytes were incubated with ciprofloxacin or levofloxacin at different concentrations (0, 3, 10, 30 and 100mg/L medium) for up to 4 days. Ultrastructural changes were studied by electron microscopy, and alterations in synthesis of specific proteins were determined using immunoblotting. At concentrations, which are achievable during quinolone therapy, 3mg ciprofloxacin/L medium significantly decreased type I collagen; similar changes were observed with 3mg ciprofloxacin or 10mg levofloxacin/L medium for the beta(1)- integrin receptors. Effects were intensified at higher concentrations and longer incubation periods. Cytoskeletal and signalling proteins, such as activated shc or erk 1/2, were significantly reduced by both fluoroquinolones already at 3mg/L. Furthermore, time- and concentration-dependent increases of matrix metalloproteinases as well as of the apoptosis marker activated caspase-3 were found. Apoptotic changes were confirmed by electron microscopy: both fluoroquinolones caused typical alterations like condensed material in the nucleus, swollen cell organelles, apoptotic bodies and bleb formation at the cell membrane. Our results provide evidence that besides changes in receptor and signalling proteins apoptosis has to be considered as a final event in the pathogenesis of fluoroquinolone-induced tendopathies.