To determine the developmental potential of 8-Cell blastomeres, we reconstituted composite embryos by introducing one blastomere of fertilized 8-Cell embryos to 4-Cell diploid parthenogenones and then cultured in vitro and transferred to foster mothers. Diploid parthenogenones were artificially induced by successive treatment of 7% ethanol and 5 ㎍/㎖ cytochalasin B. 70.7% of them developed to blastocyst in vitro and when transferred to pseudopregnant mice, they developed to 25-somite fetus at 11th pregnancy date. Next, we reconstituted composite embryos with 4-Cell diploid parthenogenones and 8-Cell fertilized embryos through a serial process composed of zona cutting, blastomere destruction, dissociation and transplantation. Then 70.5% composite developed to blastocyst in vitro and we were able to obtain fertile mice from them through embryo transfer. Especially, when they were transferred to oviduct of Day I pregnant mice at blastocyst stage, 5 of 7 transferred embryos were born to offsprings. There fore, these results indicated that each blastomere of mouse fertilized 8-Cell embryo can develop to adult if an appropriate support is introduced.