An experimental model of mandibular infection was prepared by using NZW rabbits, and the influences of the antimicrobial agents used in dentistry on polymorphonuclear leukocytes (PMNs) functions, in particular on phagocytic activity, in vitrowere investigated. Experiments were performed before and on the third and the seventh day after inoculation. Cefaclor (CCL) and cefteram (CFTM) as cephems, erythromycin (EM) and clarithromycin (CAM) as macrolides, and tosuf loxacin (TFLX) and sparfloxacin (SPFX) as newquinolones were used for the study at concentrations of 1, 10, and 100μg/ml, respectively. Phagocytosis was measured with flow cytometry and the results were as follows:<BR>1) The phagocytic activity of the PMNs of the infection model was significantly enhanced on the third and the seventh day after inoculation compared with that before inoculation.<BR>2) Neither CCL nor CFTM, cephem antibiotics, had an influence on phagocytic activity at any time including pre- and postinoculation at any of the concentrations studied.<BR>3) EM and CAM, macrolide antibiotics, had no effects on phagocytic activity before and after inoculation at the concentration of 1μg/ml. However, inhibition of phagocytosis was observed at 10 and 100μg/ml in a concentration-dependent manner.<BR>4) TFLX and SPFX, newquinolone antibiotics, had no effects on phagocytic activity at any time during the experiments including pre- and postinoculation at the concentration of 1μg/ml. Meanwhile, the activity was slightly elevated at 10μg/ml and was suppressed at 100μg/ml.