Observations of pronounced increases in the yield stress of fcc Pb crystals due to Ag doping are reported. The introduction of silver solutes in lead produces a dramatic increase in the yield strength of the crystals. This hardening effect is produced by a solute concentration too small to be associated with substitutional solutes causing the hardening such as the case of tin in lead and is more consistent with solutes which produce interstitial hardening in crystals. The extent of hardening is dependent on the amount of interstitial silver trapped in solution. Further experiments are presently being conducted to more accurately determine the concentration dependence of the yield stress in the light of an observed increased hardening. In addition the influence of interstitials on the difference in flow stress between the normal and superconducting states is also under investigations and will be reported elsewhere. (auth)