Gelatin microspheres are well-known for their capacity to release growth factors in a controlled manner. but gelatin microspheres do not calcify in the absence of so-called bioactive substances that induce deposition it calcium phosphate (CaP) bone-mineral. This study has investigated if CaP nanocrystals can be incorporated into gelatin microspheres to render these inert rnicrospheres bioactive without compromising the drug releasing properties of gelatin mierospheres: Incorporation of CaP nanocrystals into gelatin microspheres resulted into reduced biodegradation and drug release rates; whereas their calcifying capacity increased strongly compared to inert gelatin microspheres. The reduced drug release rate was correlated to the reduced degradation rate as caused by a physical cross-linking effect of CaP nanocrystals dispersed in the gelatin matrix. Consequently, these composite microspheres combine beneficial drug-releasing properties of organic gelatin with the calcifying capacity of a dispersed CaP phase.