This paper presents a new implementation of MPI on a cluster of Linux-based, dual-processor PCs interconnected by a Myricom high speed network. A survey of existing software for this hardware configuration resulted in the non-availability of a fully functional, correct and complete MPI library exploiting the full hardware potential. Our library uses MPICH for the high level protocol and FM/HPVM for the basic communications layer. It allows multiple processes and multiple users on the same PC, and passes an extensive test suite, including all test programs from the MPICH distribution, for both C and Fortran. The presented benchmarks, both simple communication kernels and full applications, show good performance. The result is the first high-performance MPI interface which allows regular multi-user service for applications on our PC cluster.