1. In an experimental system of implant-induced supernumerary limb formation in the newt, two inhibitors of limb regeneration, X-rays and beryllium nitrate, were individually applied to either the frog kidney implants or to the host limb. 2. Kidney implants irradiated with 2,220 r or 3,000 r induced normal percentages of supernumerary limbs whereas normal implants placed into limbs previously irradiated with 2,000–4,000 r did not stimulate any supernumerary growth. 3. Implants soaked in N/7 beryllium nitrate for two minutes stimulated normal percentages of supernumerary growth, but, when normal implants were placed into beryllium-treated limbs, no regenerative response was noted. 4. Neither X-irradiation nor beryllium nitrate inactivate the ‘factor’ in frog kidney which induces supernumerary limb formation. When host limbs were treated with these inhibitors, dedifferentiation was blocked and a regenerative response did not occur despite the presence of an adequate stimulus in the form of the kidney implant. It is concluded that these inhibitors disrupt some phase of the process occurring between the initial stimulus and the onset of overt dedifferenti