During the past decade, there has been much interest in Multilevel Secure (MLS) database management systems. This has resulted in several commercial MLS database systems and research prototypes available today. We believe that interoperation in MLS database systems is the next logical step. The diversity of solutions among these systems also motivates the study of the interoperability issues. A federated MLS database system is a system implemented on top of a collection of autonomous pre-existing MLS local database systems (LDBSs). Transaction processing in federated MLS database systems is complicated by autonomy and security requirements, since these requirements often conflict with each other. In this paper, we propose a concurrency control protocol for transaction processing in federated MLS database systems. Our protocol ensures global serializability but requires that the security lattice at each local site be totally ordered. However, the union of each local lattice can be partially ordered. In the future work we hope to relax the restriction on local security lattice.