The susceptibility of anaerobic bacteria to two new fluoroquinolones, PD 131628 (the bioactive form of PD 131112 or CI-990) and clinafloxacin (CI-960 or PD 127391), was determined with the agar dilution procedures and two media, and one broth microdilution procedure. Sparfioxacin and ciprofloxacin were also tested by the broth microdilution method. One hundred anaerobic isolates and four control strains were tested by the three methods which gave minimum inhibitory concentrations (MICs) that were essentially comparable, but not identical. With the broth microdilution method, the relative potency of the four fluoroquinolones was: clinafioxacin > PD 131628 > sparfioxacin > ciprofloxacin. For the latter three drugs but not clinafloxacin, the MIC values were often near the proposed interpretive breakpoint concentrations, and thus minor methodological differences frequently influenced the interpretive categories. Replicate agar dilution tests in five laboratories established MIC control limits for each of four control strains: those MIC limits could also be used to define the expected performance of the two alternative methods.