The effect of growth conditions on the antibacterial activity of DU-6859a against Staphylococcus auerus, Escherichia coli, and Pseudomonas aeruginosa was compared with those of levofloxacin, sparfloxacin, and ciprofloxacin. This activity was not affected by different media, inoculum size or the addition of human serum, but was decreased under acidic conditions, in human urine, and in the presence of magnesium and ferrous ion, as were the other quinolones tested. Time-kill curve studies demonstrated the bactericidal action of DU-6859a against S. aureus, Streptococcus pneumoniae, E. coli, and P. aeruginosa. Morphological alteration of these bacteria after exposure to DU-6859a also demonstrated its bactericidal activity. The frequency of spontaneous resistance to DU-6859a was less than or equal to those of the reference drugs.