Hyperosmolality of the urine and renal medulla inhibits polymorphonuclear leukocyte (PMN) functions such as phagocytosis, intracellular killing and superoxide generation. The main factors of hyperosmolality in the urinary tract are urea and NaCl. We studied the chemiluminescent (CL) functions of PMNs in hypersomotic urea and NaCl solutions and the effects of ofloxacin, a new quinolone antimicrobial, for inhibiting the functions of PMNs subjected to hyperosmolality. When PMNs were incubated with hypersomotic urea and NaCl solutions, the CL response was significantly reduced. The reduced CL response was significantly preserved by supplementation with ofloxacin (OFLX). The CL response of PMNs may thus also be reduced in the urinary tract and OFLX may exert strong antimicrobial and preservative effects on PMN.