Introduction: P-glycoprotein (P-gp) inhibitors are usually used to treat tumors that overexpress P-gps. However, most common types of breast cancers, such as Luminal A, are low-P-gp expressing, at least during the initial phases of treatment. Therefore, it would be interesting to know if P-gp inhibitors are still useful in treating low-P-gp-expressing tumors. Methods: In the study reported here, the human breast-cancer cell line MCF-7, chosen as a model of Luminal A, was found to be low-P-gp expressing. We designed a novel doxorubicin (DOX) sterically stabilized liposome system co-loaded with the low-dose P-gp inhibitor cyclosporine A (CsA) (DOX/CsA/SSL). Results: The co-delivery system showed good size uniformity, high encapsulation efficiency, and a desirable release profile. The cell-uptake and cytotoxicity studies demonstrated that CsA could significantly enhance the intracellular accumulation and toxicity of free DOX and the liposomal DOX in MCF-7 cells. The confocal microscopy and in vivo imaging study confirmed the intracellular and in vivo targeting effect of DOX/CsA/SSL, respectively. Finally, the in vivo study proved that DOX/CsA/SSL could achieve significantly better antitumor effect against MCF-7 tumor than controls, without inducing obvious systemic toxicity. Conclusion: This study demonstrated that the co-delivery of a low-dose P-gp inhibitor and liposomal DOX could improve the therapy of low-P-gp-expressing cancer, which is of significance in clinical tumor therapy. © 2014 Gao et al.