[en] The MCNP code is written and maintained by Group X-TM at Los Alamos National Laboratory. In response to a variety of needs, and particularly in support of the Accelerator Production of Tritium (APT) program, the authors have recently undertaken a major effort to expand the capabilities of MCNP to increase the set of transportable particles; to make use of newly evaluated high energy nuclear data tables for neutrons, protons, and potentially other particles; and to incorporate physics models for use where tabular data are unavailable. A preliminary version of the expanded code, called MCNPX, has now been issued for testing. The new code includes all existing LAHET physics modules, and has the ability to utilize the 150 MeV data libraries that have recently been released by LANL Group T-2