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ABSTRACT: The uptake of sparfloxacin (CAS 11542-93-9) by human monocytes was studied by comparison with ciprofloxacin (CAS 86393-32-0). The human monocytic THP 1 cells were incubated with the antibiotics for 2 h. Entry of antimicrobials into the cells was determined by means of a velocity gradient centrifugation technique and HPLC assay. Antibiotic uptake was expressed as the ratio of the intracellular to the extracellular drug concentration (IC/EC). Quinolones enter readily in monocytic cells but sparfloxacin is taken up more rapidly than ciprofloxacin. At steady-state the IC/EC ratio of sparfloxacin (9.07) is higher than IC/EC of ciprofloxacin (4.29). Characterization of quinolone uptake suggests that these drugs penetrate throughout the THP 1 membrane by passive diffusion. However, the results of the present study indicate that additional mechanisms may contribute to intracellular accumulation of ciprofloxacin and sparfloxacin. Gemfibrozil, an inhibitor of organic anion transport, increases the accumulation of ciprofloxacin but does not modify IC/EC of sparfloxacin. It can be concluded that human monocyte-like cells have functional organic anion transporters and that this way of secretion is quinolone selective.