Aims: Multidrug resistance (MDR) mediated by overexpression of drug efflux transporters such as P-glycoprotein (P-gp), is a major problem, limiting successful chemotherapy of breast cancer. The use of siRNA to inhibit P-gp expression in MDR tumors is an attractive strategy to improve the effectiveness of anticancer drugs. Method: We have synthesized a novel conjugate between a phospholipid (dioleoylphosphatidylethanolamine) and polyethylenimine (PEI) for siRNA delivery, for the purpose of silencing P-gp to overcome doxorubicin resistance in MCF-7 human breast cancer cells. Results: The dioleoylphosphatidylethanolamine-PEI conjugate enhanced the transfection efficacy of low-molecularweight PEI, which was otherwise totally ineffective. In addition, the polyethylene glycol/lipid coating of the new complexes gave rise to small micelle-like nanoparticles with improved biocompatibility properties. Both coated and noncoated formulations delivered P-gp-specific siRNA to MDR cells. Discussion: The combination of doxorubicin and P-gp silencing formulations led to a twofold increase of doxorubicin uptake and a significant improvement of the therapeutic effect of doxorubicin in resistant cells.