P-glycoprotein (PGP), which confers multidrug resistance to cancer cells, is expressed in mouse kidney proximal tubule and mesangium. We report on the expression of PGP and its xenobiotic transport function in mesangial cells. Studies were performed in a mouse mesangial cell line (TKGM) and two cell clones. Ribonuclease protection assay and Western blot analysis demonstrated that TKGM cells expressed mdr1 and mdr3, the isoforms responsible for multidrug resistance. TKGM-F12 cells coexpressed mdr1 and mdr3 whereas TKGM-G2 cells expressed only mdr1. The drug transport function, measured by rhodamine 123 (R-123) efflux, was smaller in TKGM-F12 than in TKGM-G2 cells. The PGP substrates adriamycin, cyclosporin A, vinblastine, and verapamil inhibited R-123 transport in TKGM and TKGM-G2 cells. In the cells studied, PGP conferred some resistance to adriamycin; concomitant exposure to adriamycin with another PGP substrate impaired cell growth. The differential expression of mdr1 and mdr3 in mouse mesangial cell clones, the ability of mdr1 PGP to transport R-123, and the impairment of PGP-mediated transport in TKGM-F12 cells, coexpressing mdr1 and mdr3 products, are demonstrated. PGP may play a physiological role in mesangial cells.