Poly-dl-lactide-poly(ethylene glycol) (PELA) block copolymers containing same the content (10%) of polyethylene glycol (PEG) were synthesized with five different molecular weight of PEG by ring-opening polymerization. PELA microspheres containing human serum albumin (HSA) were elaborated by solvent extraction method based on the formation of double w/o/w emulsion. In vitro matrix degradation and protein release of these microspheres were performed in hosphatebuffered saline (PBS) (154mM, pH 7.43). The degradation profiles were characterized by measuring the loss of microspheres mass, the decrease of polymer intrinsic viscosity, the decrease of pH value of degradation medium, the reduction of polymer number-average molecular weight (M) and the change of molecular weight polydispersity (M/M). n w n The release profiles were investigated from the measurement of protein presented in the release medium at various intervals. It showed that the matrix degradation and protein release profiles were highly polymer-dependent. The extent of burst release in the initial protein release increased with the decrease of molecular weight of PELA copolymer. It is suggested that these matrix polymers may be optimized as carriers in protein (antigen) delivery system for different purposes.