The present study was carried out to investigate the cloning of mouse embroys. Female ICR micx were superovulated by intraperitonial injection of PMSG and HCG. Two- and four-cell embryos were digested by proteolytic enzymes; pronase, trypsin, protease and pepsin, and lipase, and separated in vitro by fine glass needles with hand and isolated blastomeres were cultured in vitro for 72 hours. The results obtained were summarized as follow; 1. The proteolytic enzymes had been shown to cause dissolution of zona pellucida, but lipase didn`t. 2. In two-cell mouse embryos, the rate of dichotomy was 89.7% and 97.6 k in zona softened and removed, respectively. Among the dichotomized two-cell mouse embryos, one half and both halves undamaged blastomeres were 30.8% and 57.9% in zone softened and 12.2% and 85.4% in zona removed, respectively. 3. In four-cell mouse embryos, the rate of dichotomy was 85.3% and 96.0% in zona softened and removed. The rate of 4:0, 3:1, 2:2 and 1:3 (undamaged blastomeres vs. damaged blastomeres) were 8.8%, 15.7%, 42.2% and 18.6% in zona softened and 16.8%, 30.9%, 35.6% and 12.8% in zona removed, respectively. 4. The developmental rate of zona softened and removed single blastomeres was 68.5% and 69.0% in 2cell embryo, and 46.7% and 38.5% in 4-cell embryo, respectively. And the developmental rate of zona softened and removed single blastomeres was 10.5% and 6.9% in 2~ell embryo, and 0% and 0% in 4-cell embryo, respectively. The blastomeres isolated from 2-cell embryo developed into blastocyst, but that from 4-cell embryo developed into 8-cell stage, respectively.