These experiments were carried out to develop the technique of nuclear transplantation necessary for elevating utilization efficiency of high quality embryos and the production of clone animals. Embryos of pronucleus stages were obtained from ICR mice. Removal of pronuclei and their transfer to recipient embryos were carried out by micromanipulation and virus mediation. The results obtained in these experiments were summarized as follows ; 1. Total 337 pairs of pronuclear stage embryos were subjected to nuclear transplantatin and 247 pairs(73.3%) of them were successfully transplanted and the number of fused embryos between transplanted nucleus and cytoplasm was 188 pairs(55.8%). 2. Of the 188 fused embryos cultured in vitro, 174(92.4%), 131(69.7%) and 117(62.2%) embryos were developed to 2-cell, morula and blastocyst stages, respectively. 3. When total 104 nuclear transplanted embryos were transferred to uteri of recipient mice on day 2-3 of pseudopregnancy, 4 of 12 recipient mice were pregnant and the number of embryos developed to young was 28(26.9%).