The infection of cell lines with mycoplasma can cause severe problems as the contaminants affect virtually every cell parameter. We attempted to eliminate mycoplasma from contaminated cell lines using the fluoroquinolone antibiotic ciprofloxacin. Mycoplasma-infected cell lines were cultured with 10 micrograms/ml ciprofloxacin for 14 days. The elimination or persistence of mycoplasmal infection was monitored by diamidino-2-phenylindole (DAP) DNA staining, RNA hybridization test and broth-agar microbiological culturing. Seventeen out of 21 positive cell lines (81%) have been successfully treated using ciprofloxacin. Mycoplasma infections are unacceptable in experimental in vitro systems and require an elimination procedure of certain efficiency. The use of adequate detection methods in the routine control of cell lines and the avoidance of emerging resistant strains are of the utmost importance.