Mechanisms by which human bone resists fracture are not clearly understood. Only resistance to mode I (tension loading) crack growth has been characterized. In this investigation, mode II propagation fracture toughness of human tibia is measured, so that the fracture resistance of human bone to shear loading can be defined. The critical strain energy release rate, Gc, is found by applying experimental test data to results of finite element analysis. Average Gc for mode II testing of human bone specimens increased from 406 N/m (s.e. = 103 N/m) to 852 N/m (s.c. = 209 N/m) for specimen thickness between 1.81 and 3.14 mm. respectively. Mode II Gc was compared to values reported for mode I Gc.