The development of the early mouse embryos was investigated from 36 to 84 h after the presumed time of fertilization. The mean number of cells constituting the embryo from the intra-strain crosses of three strains at 84h of development was as follows: DDK, 22-7; BS, 43-1 and ITES, 62-0. A significance was observed in the cleavage rate of the embryo between DDK and the other two strains. The F1 embryos from the crosses of BS females X DDK males and ITES females X DDK males showed almost the same progress of development as BS and ITES embryos, respectively. Therefore, it is concluded that the early development of the F1 embryos iss, DDK females X BS males, showed conspicuously small cell numbers as compared with their litter-mates that developed to the blastocyst stage. It is inferred that the discrimination is already made during cleavage between the fortunate survivors and the lethal embryos due to the incompatibility between the cytoplasmic factor of DDK eggs and alien spermatozoa.