In Access 2000, you can compile your application code modules, which renames the file extension from .mdb to .mde (choose Database Utilities and Make MDE File from the Tools menu). This prevents editing or addition to the objects in the database, which is useful when you're distributing the application to third parties.
A side effect is that it also prevents the merging of replicated objects from the Design Master with local objects at replicas. The net result is that you can't convert a replicated database into an MDE file. To use MDE files with replication, you must convert the file to the MDE format first, and then make it replicable.