Various factors affecting the stability and release of a model protein, bovine serum albumin (BSA), encapsulated within biodegradable poly(D,L-lactide-co-glycolide) microparticles were examined in detail. Size exclusion chromatographic analysis of BSA encapsulated within microparticles revealed that the covalent aggregation of BSA significantly occurred during the encapsulation process and throughout a release study. The amount of multimeric water-soluble aggregates within microparticles increased approximately B-fold immediately after microsphere preparation, and 14-fold after 28 days of incubation in the medium, compared to that of fresh BSA. The released BSA in the medium exhibited about 50% aggregated species after 28 days of incubation. When using carboxymethylated BSA which blocks the free single thiol group in BSA, the aggregation was effectively minimized within microparticles and release medium. This provided conclusive evidence that the aggregation was largely covalent in nature. It was found that the covalent aggregates were generated in part by a free thiol group induced disulfide bond exchange reactions in BSA, however they minimally affected the BSA release kinetics. Enhanced release rate of BSA from microparticles in the release medium containing 5 mM sodium dodecyl sulfate suggested that nonspecific protein adsorption onto polymer surface played a more important role than the protein aggregation for protein release.