Non-transformed shoots of chimeric origin failed to survive in the presence of 100 mg/l kanamycin and regeneration of such shoots was completely inhibited. Similar findings were reported with kanamycin when bean callus was subjected to Agrobacterium-mediated transformation ( Franklin et al., 1993). Kanamycin is the most widely selectable marker used in plant transformation, and is an efficient selectable marker for guayule ( Dong et al., 2006). A kanamycin concentration of 50 mg/l was the minimum dose for inhibiting regeneration in untransformed HRC cultures. Transformed callus cultures were also exposed to 100 mg/l kanamycin, for more stringent selection, to completely suppress the untransformed tissues.