Nowadays, multimedia contents are invading the digital world through a wide family of distribution channels, and are played on terminals with various capabilities and resources. In such hybrid and heterogeneous environments, the collaboration between operating system, network and application layer becomes a critical requirement. The middleware is a new layer charged with the interoperability issues. Defining a multimedia middleware requires the integration of technological solutions on the middleware side (RMI, web services, J2EE, DCOM) with network protocols for transmitting multimedia content (HTTP, RTSP, SIP, H323) on different hardware platforms (PC, PDA, mobile phone). The availability of international standards offering solutions on all these levels becomes a strong requirement. MPEG-4 is a powerful multimedia standard in terms of media representation, scene composition, and user interactivity. However, building an MPEG-4 application demands an in-depth knowledge of MPEG-4 specifications which currently limits the world wide deployment of the standard. In order to facilitate MPEG-4 usability for non-expert developers, this paper addresses the technical issues related to the implementation of an MPEG-4 Software Development Kit (SDK). By focusing on the MPEG-4 scene composition level and on the graphics representation features, we build a low-level MPEG-4 SDK (scene graph access, media and stream processing) compliant with scene and media specifications. The more specific issue of virtual character animation is addressed by developing an intuitive API; it is referred to as MPEG-4 VC API and supports high-level functionalities (data-based semantic access, hierarchical object processing and stream control). We demonstrate the relevance of this toolkit for the easy design and creation of real applications by implementing a set of applications. The first one is a plug-in for 3dsmax providing MPEG-4 content importing/exporting. The second one is a converter format for Maya and MPEG-4. The third one is a plug-in for QoSmos, a QoS framework for interactive 3D applications. No particular knowledge of the MPEG-4 standard is therefore required. The SDK toolkit is extensively evaluated within the OLGA IST FP6 European Project