Short-term and long-term cytotoxicity of four fluoroquinolones (ciprofloxacin, rufloxacin, ofloxacin, lomefloxacin) on 7 established murine cell lines (WEHI-3B, L1210, EL4, P388D1, 32DC13, L929, SR-4987) by a microtiter MTT assay have been studied. In short-term cytotoxic test (24 hr), cell lines with a high proliferating cell rate (as leukemias) showed a greater sensitivity to quinolones than other cell lines. In long-term cytotoxic test (7 days) no different sensitivity was observed among the cell lines. In short-term test ciprofloxacin and rufloxacin were more toxic than lomefloxacin and ofloxacin whereas in the long-term test the activity of the four quinolones was similar. Ratio between IC50 on cell lines and MIC50 against gram negative bacteria evidenced remarkable differences when short-term or long-term cytotoxic tests were considered. The results confirm toxic activity of quinolones on mammalian cells evidencing that the sensitivity to quinolones, in short-term cytotoxic test, correlates with the doubling time of cell population. The results further suggest that long-term cytotoxic tests measure better the antiproliferating activity of quinolones providing a more powerful assay to investigate their in vitro toxicity.