Improved biopharmaceutical delivery may be achieved via the use of biodegradable microspheres as delivery vehicles. Biodegradable microspheres offer the advantages of maintaining sustained protein release over time whilst simultaneously protecting the biopharmaceutical from degradation. Particle samples produced by ultrasonic atomization were studied in order to determine a feed stock capable of producing protein loaded poly-epsilon-caprolactone (PCL) particles suitable for nasal delivery (i.e., less than 20 mu m). A 40 kHz atomization system was used with a 6 turn full wave atomization probe. The effect of solids percent, feed flow rate, volumetric ratio of the polymer stock to the protein stock, and protein concentration in the protein stock on particle size characteristics were determined. It was shown that feed stocks containing 100 parts of 0.5 or 1.0% w/v PCL in acetone with one part 100 mg ml(-1) BSA and 15 mg ml(-1) PVA produced particles with a mass moment diameter (D[4,3]) of 13.17 mu m and 9.10 mu m, respectively in addition to displaying high protein encapsulation efficiencies of 93 and 95%, respectively. The biodegradable PCL particles were shown to be able to deliver encapsulated protein in vitro under physiological conditions.