BIGNUM: a portable and efficient package for arbitrary-precision arithmetic. We describe a C package for arbitrary-precision integer arithmetic that is portable, yet efficient. Making the package run fast on a given computer involves re-writing a small kernel of our package in native assembly language. We provide such assembly code for VAX, 68020 and NS instruction sets, with good benchmarks. This package serves as a foundation for two arithmetic packages written in higher level languages LeLisp and Modula2