Biodegradable polymer poly-D,L-lactide-polyethylene glycol (PLA-PEG) was synthesized with stannous octoate (SnOc(2)) as catalyst by a cationic ring-opening polymerization. The molecular weight of PLA-PEG is the highest at a content of 0.1% SnOc(2). The PLA-PEG microspheres carrying protein were prepared by a solvent evaporation composite emulsion technique with a narrow size range (1-2 mu m). The sizes of PLA-PEG microspheres increased with the increase of the molecular weight of PLA-PEG. The PEG in PLA-PEG (10%) significantly improved the size control of the microspheres of the PLA family as a drug carrier matrix. Polyvinyl alcohol (PVA) aqueous solution was used as dispersion medium for microsphere preparation. The concentration of the PVA solution can affect the size of the PLA-PEG microspheres. The differential scanning calorimetry data showed that the PLA-PEG microspheres can efficiently encapsulate the protein and that the crystalline of the microspheres carrying protein was lower than that of the nonprotein-loading microspheres. The amount of protein carried in the PLA-PEG microspheres was related to the nature of the protein itself.