An X-irradiated newt limb is able to regenerate if non-irradiated skin as well as non-irradiated muscle is transplanted to the stump. Non-irradiated epidermis is brought to the stump with a skin graft but not with a muscle graft. In order to know whether limb regeneration required healthy epidermis or not, a triploid skin cuff was set at the most proximal level of an irradiated limb and muscle was transplanted to the level of the midforearm. The forearm was then amputated through the muscle graft. A cytophotometrical analysis of DNA content of the epidermis cell nuclei sampled from the skin of the regenerate was undertaken to detect a migration of triploid epidermal cells. The result was a complete replacement of diploid irradiated epidermis by triploid epidermis, during the six weeks necessary for regeneration. Another investigation consisted of detecting a possible migration of non-irradiated triploid epidermis along an irradiated limb which had not been amputated. Healthy epidermis was found to migrate distally and replace irradiated epidermis in three weeks. Previous experiments involving transplantation of a non-irradiated skin cuff or muscle to an irradiated limb stump were carried out again but on animals which had been entirely irradiated to prevent any extra healthy epidermis cells from contaminating the regenerating limb epidermis. A regenerate developed from the skin graft but not from muscle graft. It is concluded that healthy epidermis must be present on the limb stump to permit the blastema to develop.