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Desirably porous biodegradable hybrid composite microspheres were fabricated for use in bone graft and bone substitute applications. In this study, novel poly(ε-caprolactone)/biphasic calcium phosphate (70/30) composite microspheres (PCL/BCP MPs) were prepared using the emulsion solvent-evaporation method. Throughout this process, the ammonium bicarbonate (NH₄HCO₃) content was changed to obtain the desired porous structure. However, to maintain the spherical shape, the NH₄HCO₃ content should not be higher than 5%. In the optical images of the PCL/BCP MPs, almost all the microparticles had a spherical shape, and the average diameter was about 600 μm. The scanning electron microscopy and cross-sectional optical images showed that the pore density and pore diameter of PCL/BCP MPs increased with increasing initial NH₄HCO₃ concentrations. In the phase-composition analysis of the PCL/BCP MPs, which was characterized by X-ray diffraction and EDS, the two crystals BCP and PCL phases were shown to be miscible in PCL/BCP MPs. When the degradation of these microspheres was characterized, PCL/BCP MPs-0, PCL/BCP MPs-2, and PCL/BCP MPs-5 were found to display a sustained biodegradability, and the rate of degradation increased at higher initial NH₄HCO₃ concentrations. Proliferation of cells on three different sample types was assessed and compared, and based on these results, the PCL/BCP MPs-5 was chosen to study MG-63 osteoblast-cell adhesion, growth, and proliferation. Furthermore, confocal images indicated that the cells effectively adhered, spread, and proliferated on PCL/BCP MPs-5 during a 5-day culture period., (Copyright © 2011 Wiley Periodicals, Inc.)