Clinafloxacin and trovafloxacin are two new fluoroquinolones for which few comparative data are available. When MICs of ciprofloxacin against Gram-positive and Gram-negative nosocomial species were compared, clinafloxacin was the most potent although trovafloxacin was also more active than ciprofloxacin against Staphylococcus aureus and enterococci. All three drugs were bactericidal. Clinafloxacin displayed the lowest frequency of resistance, approximating 10(-11). Development of resistance studies over 13-14 passages in the presence of drug revealed a 32-fold increase in MIC of clinafloxacin against S. aureus compared with 512- and 1024-fold for trovafloxacin and ciprofloxacin respectively, although the three drugs were comparable against Enterococcus faecalis and the Gram-negative bacilli.