We discuss laboratory tests of a prototype in situ geotechnical torsional cylindrical impulse shear testing system. The purpose of the torsional cylindrical impulse shear test is to provide estimates, for soil deposits, of in situ undegraded nonlinear inelastic shear stress versus strain characteristics. The laboratory tests were conducted on a large sample of medium dense Ottawa sand. Undegraded nonlinear inelastic shear stress versus strain characteristics estimated for the sand based on results from these tests are consistent with corresponding published estimates based on results from laboratory tests (resonant column and cyclic triaxial tests) and field array studies. Shear strains of up to about 0.1% and an 80% reduction in shearing stiffness were inferred to have developed. Test results were judged to be sensitive to undergraded soil characteristics (low strain shear modulus and shape of shear stress versus strain curve) and repeatable. Also, we judged that penetration-induced disturbances to the test sand were not excessive. Major difficulties or limitations of the testing system were not encountered during the laboratory tests. We concluded that the torsional cylindrical impulse shear test is promising for field use.