BACKGROUND AND OBJECTIVE: The inability of radiotherapy to eradicate completely certain human tumours may be due to the presence of resistant hypoxic cells. Several studies have confirmed the radiosensitizing effect of paclitaxel, a microtubular inhibitor. The object of this study was to evaluate the physicochemical characteristics of paclitaxel-loaded nanoparticles, and determine the ability of the released paclitaxel to radiosensitize hypoxic human breast carcinoma cells (MCF-7) with respect to radiation dose. METHODS: The poly(d,l-lactide-co-glycolide) (PLGA) nanoparticles containing paclitaxel were prepared by o/w emulsification-solvent evaporation method. The morphology of the paclitaxel-loaded nanoparticles was investigated by scanning electron microscopy. The drug encapsulation efficiency (EE) and in vitro release profile were measured by high-performance liquid chromatography. Cell cycle was evaluated by flow cytometry. Cell viability was measured by the ability of single cells to form colonies in vitro. RESULTS: The prepared nanoparticles were spherical with diameter between 200 and 800 nm. The EE was 85.5%. The drug release pattern was biphasic with a fast release rate followed by a slow one. Co-culture of human breast carcinoma cells (MCF-7) with paclitaxel-loaded nanoparticles demonstrated that released paclitaxel retained its bioactivity to block cells in the G2/M phase of the cell cycle and effectively sensitized hypoxic MCF-7 cells to radiation with radiosensitivity shown to be dependent of radiation dose at levels of dosages studied. The sensitizer enhancement ratio for paclitaxe-loaded nanoparticles at 10% survival is approximately 1.4. CONCLUSION: This work has demonstrated that paclitaxel can be effectively released from a biodegradable PLGA nanoparticle delivery system while maintaining potent combined cytotoxic and radiosensitizing abilities for hypoxic tumour cells.