The in vitro activities of eight quinolones against 115 coryneform bacteria (20 Corynebacterium jeikeium, 15 Corynebacterium minutissimum, 15 Corynebacterium striatum, 25 Corynebacterium urealyticum, 10 Corynebacterium xerosis, 10 Corynebacterium group ANF-1, 10 Corynebacterium group 12, and 10 Listeria monocytogenes) were determined. The MICs of ciprofloxacin, ofloxacin, and sparfloxacin for 90% of C. jeikeuim, C. urealyticum, and C. xerosis isolates tested were >16 mu g/ml. Those of BAY Y 3118 and clinafloxacin against these species were 0.5 and 1 to 2 mu g/ml, respectively. The MICs for 90% of all 115 strains tested were 0.5 mu g/ml for BAY Y 3118, 1 mu g/ml for clinafloxacin, 2 mu g/ml for E-5068, 4 mu g/ml for E-5065, and >16 mu g/ml for ciprofloxacin, ofloxacin, sparfloxacin, and E-4868.