Poly(D,L-lactide-co-glycolide) microspheres containing BSA were prepared by a modified solvent evaporation method using a double emulsion. These microspheres were characterized for size, morphology, surface absorbed protein, encapsulation efficiency and release kinetics. The influence of two formulation variables (the procedure to obtain the first emulsions and the lyophilization of the microspheres once obtained) on the physical characteristics and release behaviour of the microspheres was also investigated. Sonicated microspheres were smooth and spherical, with a mean particle size of 20 microns and an encapsulation efficiency of 81%. When the first emulsion was prepared by vortex mixing the particles were irregular and porous, with a mean size of 31 microns and a lower encapsulation efficiency (56%). The sonication allows a more homogeneous emulsion as well as a lower percentage of albumin adsorbed on the surface. The in vitro release profile was described as a biexponential process with an initial burst effect due to the release of the protein adsorbed on the microsphere surface and a second sustained release phase due to protein diffusion through the channels or pores formed in the polymer coat. The release of BSA was dependent on the preparation method. The greatest burst release was found for microspheres formulated using the vortex mixer, 58% of the encapsulated protein was released during the first 24 h, whereas sonicated microspheres released 32.2%. This burst effect could be reduced by lyophilizing the microspheres following their preparation. The amount of protein released decreased to 28.3% and 51.6% in sonicated and non-sonicated microspheres respectively, when they were lyophilized.