Metal ions, which are present in ocular fluids, can significantly decrease the bioavailability of fluoroquinolones. This in vitro study investigated the bioavailability of ofloxacin and ciprofloxacin in the presence of two balanced salt solutions, BSS-Plus and AMO Endosol, commonly used as irrigants in cataract surgery. Minimum inhibitory concentrations (MICs) and antibiotic susceptibility were determined through broth dilution methods and a standard disk diffusion bioassay with Escherichia coli. Although neither solution affected the in vitro antimicrobial activity of ofloxacin, both markedly decreased that of ciprofloxacin. In BSS-Plus and AMO Endosol, the MICs of ciprofloxacin for several ocular isolates were increased, indicating decreased susceptibility to that drug; the activity of ciprofloxacin against E. coli was also decreased. Because the composition of BSS-Plus is similar to that of human aqueous humor, the clinical efficacy of ofloxacin may be unaffected by the ionic composition of the aqueous humor. In contrast, because the antimicrobial activity of ciprofloxacin was diminished in BSS-Plus, the clinical efficacy of this fluoroquinolone against deep ocular infections may not be as great as indicated by standard laboratory antibiotic susceptibility testing.