The flow stress of a series of KBr&sngbnd;KCl mixed crystals has been measured as a function of radiation damage. The magnitude of the increase in flow stress is approximately the same as it is for pure KCl or KBr for equivalent amounts of damage. This result shows that the radiation hardening and alloy hardening are additive. The data have also been used to indicate the size and shape of the interstitial clusters in these mixed crystals.