The purpose of this work was to develop an effective delivery system for antiangiogenic therapy. Endostatin was microencapsulated into poly(lactic-co-glycolic acid) (PLGA) microspheres by using a w/o/o multiple emulsification-evaporation technique. Endostatin microspheres showed the encapsulation efficiency 100% with mean particle size about 25 microm. Endostatin released in vitro from PLGA microspheres were biologically active and significantly inhibited the migration of endothelial cells. In rats, endostatin microspheres produced a sustained release process in which the steady-state concentration was reached from day 5 to day 27 with the steady-state levels of endostatin between 174.8+/-33.3 and 351.3+/-126.3 ng/ml. In Lewis lung cancer model, a dose of 10 mg/kg endostatin microspheres was just as effective in suppressing tumor growth as a dose of 2 mg/kg/day free endostatin for 35 days (total dose 70 mg/kg). These results indicated PLGA microspheres further reduced the amount of endostatin needed to achieve significant tumor inhibition in mice when compared with systemic administration.