Spin-dependent tunneling through magnetic junctions is sensitive to material properties near the interface and in the barrier. Results of calculations are presented showing how electron transmission through a point contact tunnel junction is affected by atomic disorder in the barrier. Giant variations in the transmission probability are found with limited disorder. Sharp peaks appear in the tunnel current when defects exist at positions in the barrier that facilitate electron hopping across the contact. Consequences for thin film tunnel junction devices are also discussed, with reference to experiments showing strong spatial variations in tunnel current. (C) 2003 American Institute of Physics.