In conventional tensile or compressive tests of single crystals, plastic deformation by slip occurs nonuniformly owing to slip plane rotation and constraint of the specimen’s ends. To minimize these problems, a ‘‘load fixture’’ (LF) that employs an air bearing and magnetic field (to provide for free rotation of the ends of the specimen) has been designed and built. The frictionless air bearing minimizes the tendency of the system to develop internal torques during the deformation of single crystals, particularly those with low critical resolved shear stresses. The LF has been tested in tensile and compressive loading of mercuric iodide single crystals. By keeping the center of rotation on the ‘‘boundary of slip’’ (i.e., on the slip plane that separates slipped from unslipped material in the plastically deformed crystal), the line of action of the applied force is maintained coincident with the sample’s central axis. The LF is particularly useful for testing small, ductile crystals and for studying mechanical hysteresis.