The penetration of a new quinolone (BAY Y 3118) into human polymorphonuclear leukocytes (PMNs) was evaluated by a fluorometric assay. The cellular concentration-to-extracellular concentration (C/E) ratio was higher than 6.3 at extracellular concentrations ranging from 2 to 100 mg/liter. The uptake of BAY Y 3118 was rapid, reversible and nonsaturable. The intracellular penetration of BAY Y 3118 was significantly affected by environmental temperature (C/E ratio at 4 degrees C, 5.4 +/- 0.5; control, 7.5 +/- 0.9; P < 0.05) and cell viability (C/E ratio in dead PMNs, 5.5 +/- 0.8; control 7.5 +/- 0.9; P < 0.05), but it was not affected by metabolic inhibitors. The ingestion of opsonized zymosan or opsonized Staphylococcus aureus significantly decreased the levels of PMN-associated BAY Y 3118. Cell stimulation by a membrane activator, however, significantly increased the intracellular concentration of this quinolone. At therapeutic extracellular concentrations (0.5, 2, and 5 mg/liter), BAY Y 3118 showed intracellular activity greater than that of ciprofloxacin against S. aureus in human PMNs. It was concluded that BAY Y 3118 reaches high intracellular concentrations within human PMNs and remains active intracellularly.