Linear elastic fracture mechanics was used to study longitudinal crack propagation in bovine compact bone. The resistance to crack initiation in slotted specimens was examined by measuring both the critical stress intensity factor, Kc, and the critical strain energy release rate, Gc. A change in specimen thickness (from 0.185 to 0.380 cm) did not affect the value of Kc (or Gc). A significant positive correlation (P < 0.01) was found between Kc (or Gc) and dry density. A comparison of the experimentally determined effective modulus (relating Kc and Gc) with an effective modulus calculated from the transversely isotropic model for bone showed good agreement.