The objective of this work was to evaluate the suitability of in situ forming biodegradable microparticles for the controlled delivery of proteins. Particular goals were: a) to evaluate the flexibility of the release patterns of in situ implant and in situ microparticle formulations using hen egg white lysozyme as model protein; b) to characterize, if and how the integrity of the model protein is affected by formulation ingredients, the manufacturing process, storage of the ISM and the release from the formulations; c) to optimize protein delivery from in situ forming implant and microparticle systems, with regard to shape and completeness of the release; d) to find / validate reliable methods allowing to extract and thus to characterize the model protein in biodegradable in situ formulations.