The multidrug resistance proteins P-glycoprotein (P-gp), breast cancer resistance protein (Bcrp), and multidrug resistance-associated protein 2 (Mrp2) are the three major canalicular transport proteins responsible for the biliary excretion of most drugs and metabolites. Previous in vitro studies demonstrated that P-gp transported macrolide antibiotics, including spiramycin, which is eliminated primarily by biliary excretion. Bcrp was proposed to be the primary pathway for spiramycin secretion into breast milk. In the present study, the contributions of P-gp, Bcrp, and Mrp2 to the biliary excretion of spiramycin were examined in single-pass perfused livers of male C57BL/6 wild-type, Bcrp-knockout, and Mrp2-knockout mice in the presence or absence of GF120918 (GW918), a P-gp and Bcrp inhibitor. Spiramycin was infused to achieve steady-state conditions, followed by a washout period, and parameters governing spiramycin hepatobiliary disposition were recovered by using pharmacokinetic modeling. In the absence of GW918, the rate constant governing spiramycin biliary excretion was decreased in Mrp2(-) knockout mice (0.0013 +/- 0.0009 min(-1)) relative to wild-type mice (0.0124 +/- 0.0096 min(-1)). These data are consistent with the approximately 8-fold decrease in the recovery of spiramycin in the bile of Mrp2-knockout mice and suggest that Mrp2 is the major canalicular transport protein responsible for spiramycin biliary excretion. Interestingly, biliary recovery of spiramycin in Bcrp-knockout mice was increased in both the absence and presence of GW918 compared to wild-type mice. GW918 significantly decreased the rate constant for spiramycin biliary excretion and the rate constant for basolateral efflux of spiramycin. In conclusion, the biliary excretion of spiramycin in mice is mediated primarily by Mrp2 with a modest P-gp component.