To analyze the mechanism of drug transport, mechanism of inhibitors, and physiological substrates of human P-glycoprotein, we established a transepithelial transport system by introducing MDR1 cDNA into LLC-PK1, a pig kidney epithelial cell line. P-glycoprotein functions as a steroid transporter as well as a drug transporter as physiological functions. P-glycoprotein also transports MDR modulators such as cyclosporin A, FK506, and calcium channel blockers.