We have built a portable platform for running Standard ML of New Jersey programs on multiprocessors. It can be used to implement user-level thread packages for multiprocessors within the ML language with first-class continuations. The platform supports experimentation with different thread scheduling policies and synchronization constructs. it has been used to construct a Modula-3 style thread package and a version of Concurrent ML, and has been ported to three different multiprocessors running variants of Unix. This paper describes the platform's design, implementation, and performance.