Nuclei from keratinized skin cells of adult Xenopus foot-webs have been transplanted to enucleated eggs of the same species. 2. The cells used to provide donor nuclei were obtained as a monolayer outgrowth from cultured foot-web explants. When explants were cultured without plasma for 3 days, over 99-9% of the outgrowth cells contained keratin as revealed by the binding of monospecific fluorescent antibody prepared against purified Xenopus keratin. Nuclei were transplanted from cells which had been cultured for 31/2 days. 3. None of the first transfer embryos developed as far as tadpoles. Eleven clones of embryos were prepared from the nuclei of partial first-transfer blastulae by use of serial nuclear transplantation. Eight of these clones contained swimming tadpoles with functional muscle and nerve cells, and six clones contained tadpoles with beating hearts, well differentiated eyes, and other organs. 4. To prove that the nuclei of nuclear-transplant tadpoles were derived from the transplanted skin cell nuclei and not from a failure of ultraviolet light to inactivate the recipient egg nucleus, 1-nu skin cell nuclei were transplanted to eggs laid by 2-nu frogs. Several advanced tadpoles from six clones were analysed for nucleolar and chromosome number and found to be 1-nu diploids. 5. The six clones of advanced tadpoles which were proved to carry the donor nuclear marker represent six first-transfer nuclei in a total sample of 129 skin cell nuclei originally transplanted. The probability that all six nuclei were derived from the 0-1% of the donor cell population not proved to contain keratin is less than one in 10(10). 6. We conclude that cell specialization does not involve any loss, irreversible inactivation or permanent change in chromosomal genes required for development.