This paper describes the developmental potential of blastomeres isolated 4-cellembryos in mice.Four-cell embryos were collected from superovulated CF #1 females which weremated with C57BL/6 males. Zona pellucidae were removed in Hanks solution containing 0.2% pronase. After removal of zona pellucida, 4-cell embryos were separeatedinto individual blastomers by pipetting in Ca⁺⁺and Mg⁺⁺free Hanks solutioncontaining 0.02% EDTA. Separated blastomeres (one-quarter embryos) were culturedin drops of M16 medium covered with paraffin oil at 37°C in an atmosphere of 5% CO₂, 95% air for 72 hours. Four one-quarter embryos developed to blastocysts were transferred into rightuterine horn of CF#1 recipients on Day 4 or 5 pseudpregnancy, and 4 normalblastocysts of CF#1 strain cultured from 4-cell stage were transferred intoleft uterine horn of the same recipient.The results were as follow: 1) The proportion of one-quater embryos developed intoblastocysts were 79/124 (63.7%). 2) By transfer of one-quater embryos developed toblastocysts, the proportion of implantation sites and live fetuses were 6/24 (25.0%) and 0/24 (0%). On the other hand the propotion of implantation sites and livefetuses were 19/24 (79.2 %) and 14/24 (58.3%).