The penetration of an optically active ofloxacin isomer [(-)-ofloxacin] into human neutrophils and different tissue culture cells (HEp-2, McCoy, MDCK, and Vero) was studied and compared with that of ofloxacin by a fluorometric assay. The cellular-to-extracellular-concentration ratios (C/E) of (-)-ofloxacin were always higher than 6, significantly greater than those of ofloxacin at extracellular concentrations of 5 and 10 mg/liter. The penetration of (-)-ofloxacin and ofloxacin was doubled when neutrophils were stimulated by phorbol myristate acetate but not affected after ingestion of opsonized Staphylococcus aureus. The C/E ratios of (-)-ofloxacin and ofloxacin for different tissue culture epithelial cells and fibroblasts were lower than those of neutrophils but still higher than 2. Both compounds produced a significant reduction in viable intraphagocytic S. aureus during 3 h of exposure to antimicrobial agents. We conclude that (-)-ofloxacin appears to reach higher intracellular concentrations than ofloxacin, remaining active inside the neutrophils.