MICs of the new fluoroquinolone, moxifloxacin, and those of ciprofloxacin, ofloxacin and sparfloxacin for 19 genetically characterized fluoroquinolone-resistant strains were determined by the agar dilution method. The MICs of moxifloxacin for Escherichia isolates with one mutation in gyrA (corresponding to Ser83-->Leu or Asp87-->Gly substitution) were 0.25-0.5 mg/L, while those of ciprofloxacin, ofloxacin and sparfloxacin were 0.06-0.25, 1 and 0.12-0.5 mg/L, respectively. These values were four- to 16-fold higher than those of the same antibiotics for the wild-type strain, E. coli KL16. Similar results were observed with clinical isolates of Salmonella spp. harbouring one mutation in gyrA leading to the substitution of Ser83 by Phe or Tyr. In the presence of two mutations in the E. coli gyrA gene, the MICs of moxifloxacin ciprofloxacin, ofloxacin and sparfloxacin were 2, 0.5, 4 and 1 mg/L, respectively; these were 32 times higher than the MICs of these agents for E. coli KL16. The MICs of the four quinolones for triple mutants with two mutations in gyrA and one in parC were even higher, i.e. 8, 8, 16 and 8-16 mg/L, respectively. The MICs of moxifloxacin for Campylobacter coli and Campylobacter jejuni strains with a gyrA mutation leading to Thr86-->Ile substitution ranged from 1 to 2 mg/L, while the MICs of ciprofloxacin, ofloxacin and sparfloxacin were 16-32 mg/L, 8-16 and 4-8 mg/L, respectively. For high-level ciprofloxacin-resistant (MICs of 32 mg/L) clinical isolates of Enterococcus faecalis with one substitution at position 83 in GyrA (E. coli coordinates), the MICs of moxifloxacin, ofloxacin and sparfloxacin were 8-16, > or = 128 and 32 mg/L respectively. In conclusion, moxifloxacin and other fluoroquinolones exhibit cross-resistance against aerobic gram-negative bacilli and enterococci. The in-vitro activity of moxifloxacin was greater than that of ofloxacin and slightly less than that of ciprofloxacin and sparfloxacin against Enterobacteriaceae, but greater than those of the three other compounds tested against Campylobacter spp and E. faecalis.