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Although chemotherapeutic nanoparticles would confer various advantages, the majority of administrated nanoparticles are known to be spoiled by the reticuloendothelial system (RES). Intending to more effectively deliver therapeutic nanoparticles to target regionsin vivo, host RES, especially Kupffer cells in the liver, have been depleted ahead of drug administration. To demonstrate this hypothesis, clodronate liposomes were preinjected into BALB/c nude mice for depletion of Kupffer cells 2 days before, and pegylated liposomal doxorubicin (Doxil) at the doses of 1.25, 2.5 and 5.0 mg/kg was administered. As a result, doxorubicin accumulation in the liver was decreased from 36 to 26% injected dose/organ by the Kupffer cells depletion, and consequently, the plasma concentration of doxorubicin was significantly enhanced threefold (from 11 to 33 μg/mL) on day 1 at 1.25 mg/kg-dose group. Doxorubicin accumulation in the tumor was increased from 0.78 to 3.0 μg/g-tissue on day 3, and tumor growth inhibition by Doxil was significantly boosted (tumor volumes from 751 to 482 mm3 on day 24) by the Kupffer cells depletion. In conclusion, Kupffer cells depletion by clodronate liposomes enhanced the plasma concentration and antitumor effects of Doxil, and would be widely applicable for various clinical cancer chemotherapies using nanoparticles.