A testing machine has been designed which provides means for investigating the performance of hollow circular cylindrical bone specimens under the combined stresses produced by tension, torsion and internal pressure. The apparatus is capable of exerting an axial force of 2,000 lbs. in tension, 960 in.-lbs. in torsion, and 8,000 psi of internal pressure. The positive displacement system of the machine delivers a minimum of 1.25 inches The capacities of the apparatus in the torsional mode are a minimum of 45° of rotation and a maximum of 150°. The application of the loads is accomplished through a nitrogen pressure cylinder with hydraulic constant rate control. The rate of travel of thecylinder may be varied to investigate the viscoelastic properties of the bone specimens. Proportionality of all three modes of deformation is maintained throughout the operation of the apparatus.