In this study, the transport characteristics of ciprofloxacin (CPFX) were investigated in a LLC-PK1 kidney epithelial cell line. CPFX uptake from the apical medium into LLC-PK1 cells on plastic dishes was shown to be temperature-dependent. Guanidine and cimetidine inhibited the uptake of CPFX, whereas tetraethylammonium chloride (TEA) and N1-methylnicotinamide (NMN) did not. CPFX transport across LLC-PK1 cell monolayers cultured on permeable supports was about 1.8 times larger in the basolateral-to-apical direction than in the apical-to-basolateral direction. Both the basolateral-to-apical and apical-to-basolateral transport of CPFX were inhibited by guanidine, whereas CPFX transport in both directions was not inhibited by TEA, NMN, or cimetidine. The basolateral-to-apical transport of CPFX was sensitive to the pH of the apical medium, and increased in an acidic pH. Enoxacin (ENX) as well as guanidine, inhibited the basolateral-to-apical transport of CPFX, and the inhibitory effect of ENX was sensitive to the pH of the apical side of the monolayers.