The in vivo efficacy of doxorubicin (DOX)-loaded poly(gamma-benzyl L-glutamate)-block-hyaluronan (PBLG(23)-b-HYA(10))-based polymersomes (PolyDOX) was evaluated. Samples were efficiently labeled with technetium-99m radionuclide with good stability for in vivo studies. PolyDOX enhanced circulation time compared to free DOX. Biodistribution studies revealed selective accumulation of PolyDOX in the Ehrlich ascites tumor (EAT) as a result of passive accumulation and active targeting (CD44-mediated endocytosis) in EAT-bearing mice. Toxicity studies demonstrated PolyDOX is a safe drug carrier, and no hemolysis was observed with PolyDOX equivalent to 200 mu g/mL of free DOX. PolyDOX dominantly controlled tumor growth by delaying doubling time of EATs compared to free DOX over 30 days after treatment. PolyDOX also increased life span six times more than free DOX. Hence, it is reasonable to expect that higher DOX levels attributable to PolyDOX improve the therapeutic index and reduce side effects due to site-specific drug accumulation.