JPLEX, short for Java simPLEX, is an automated test assembly (ATA) program. It is a mixed integer linear programming (MILP) solver written in Java. It reads in a configuration file, solves the minimization problem, and produces an output file for postprocessing. It implements the simplex algorithm to create a fully relaxed solution and branch-and-bound (BB) method to find an integer solution (Bazaraa, Jarvis, & Sherali, 1990). The input configuration file format for linear programming (LP) modeling (van der Linden, 2005) is tailored for ATA and easy to build without the help of other modeling languages. It was originally designed for midsized test assembly applications, but there is no restriction on the number of decision variables. This solver might be useful for research students who need to assemble a test from an item pool and for those who want to learn or teach LP concepts.