Due to their ever-growing success in the development of distributed applications, today's multithreaded environments have to be highly portable and efficient on a large variety of hardware. Most of these environments have an implementation built on top of standard communication interfaces such as PVM or MPI, which are widely available on existing architectures. Obviously, this approach ensures a high level of portability. However, we show in this paper that these communication interfaces do not meet the needs of RPC-based multithreaded environments as far as efficiency is concerned. The contribution of this paper is to propose a new portable and efficient communication interface, called MADELEINE, that is especially designed for such multithreaded environments. We report on several implementations of MADELEINE on top of various network protocols that demonstrate the efficiency of our approach.