Zygotes have not been recognized as nuclear recipients since enucleated zygotes receiving nuclei from beyond two-cell stage embryos are not able to form blastocysts. In the present study, a new technique of zygote enucleation is presented, which consists in selectively removing the nuclear membrane with genetic material of pronuclei, but leaving other pronuclear components in the cytoplasm. With selective enucleation it is possible - after transfer of eight-cell stage nuclei - to obtain 70.5 and 7.8% of preimplantation and full-term development respectively. Origin of cloned mice from introduced nuclei was confirmed by the coat colour and glucose phosphate isomerase (GPI) isozyme of the donor. We suggest that some pronuclear factors - taken away from the zygotes in the karyoplasts upon classical enucleation - are needed to reprogram the introduced nuclei.