['To test the hypothesis that particle size influences the magnitude of immune response produced by hepatitis B surface antigen (HBsAg) encapsulated in poly (lactic-co-glycolic acid) (PLGA) microspheres.', 'Microspheres were prepared by a double-emulsion-solvent-evaporation method, and the particles were characterized for size, morphology, porosity and antigen content. Immunogenicity of encapsulated antigen and safety were studied in rats. Uptake of fluorescent-labeled particles by rat alveolar macrophages was studied by confocal microscopy.', 'With increasing internal aqueous phase (IAP) volume of the microsphere, an increase in particle size and a decrease in particle density were observed. Particles with varying geometric diameters showed aerodynamic diameters between 1 and 6 mu. Addition of poly vinyl alcohol to the IAP resulted in particles with a porous surface. The integrity of HBsAg was maintained upon encapsulation in microspheres. Continuous release of the antigen was observed for formulations incubated in phosphate-buffered saline for 28 days. Immunogenicity increased as a function of particle size upon pulmonary administration. HBsAg encapsulated in approximately 5 mum particles elicited a significantly higher immune response compared to that encapsulated in approximately 12 mum particles. Similar to in vivo immune response data, fluorescent-labeled microspheres of smaller size were taken up more efficiently by rat alveolar macrophages compared to those of larger size. No significant increase in either tumor necrosis factor alpha level in bronchoalveolar lavage fluid or wet lung weight, indicators of inflammation, was observed in rats that received optimized formulations via the pulmonary route.'] Does particle size influence the immune response produced by hepatitis B vaccine formulated in inhalable particles?