The Secure Virtual Enclaves (SVE) collaboration infrastructure allows multiple organizations to share their distributed application objects, while respecting organizational autonomy over local resources. The infrastructure is transparent to applications, which may be accessed via a web server, or may be based on Java or Microsoft's DCOM. The SVE infrastructure is implemented in middleware, with no modifications to COTS operating systems or network protocols. The system enables dynamic updates to security policies to support changes in both coalition membership and participants' perception of risks. While the prototype demonstrates fine-grained access control for secure collaborative computing, we have identified significant issues that remain to be addressed, particularly in the area of policy development, before such collaboration will be convenient. The SVE infrastructure offers a platform and conceptual basis for further exploration of these issues and experimentation with new solutions.