The developmental potencies of melanophores, ciliated epidermal cells and cells from an established line were tested by means of nuclear transplantation into enucleated Xenopus eggs. Donor nuclei from ciliated epidermal cells of hatching tadpoles never gave rise, after transplantation, to development beyond the blastula stage; nuclei from non-ciliated cells of the same larvae, on the other hand, could fully replace a zygote nucleus. Melanophore nuclei taken from the tailfin of advanced larvae (stage 56) gave rise to blastulae and, in one case, an incipient invagination was observed. Melanophore nuclei taken from tissue cultures, set up from hatching tadpoles, proved to be competent for leading development as far as the heartbeat stage, (stage 33/34), the embryo showing well-elaborated eye Anlagen, muscles, notochord, cement gland, etc. Nuclei from the highly aneuploid established cell-line A-8 were able to give rise to hatching-stage embryos. These results suggest that some condition, provided by culturing, influences the differentiated state so as to improve the chances of the donor nucleus co-operating with the egg’s developmental programme. The important change might be the state of proliferation attained in cultured cells.