We have designed a portable interface between shared--memory multiprocessors and Standard ML of New Jersey. The interface is based on the conventional kernel thread model and provides facilities that can be used to implement user--level thread packages. The interface supports experimentation with different thread scheduling policies and synchronization constructs. It has been ported to three different multiprocessors and used to construct a general purpose, user--level thread package. In this paper, we discuss the interface and its implementation and performance, with emphasis on the Silicon Graphics 4D/380S multiprocessor.