The phenomenon of the developmental arrest at the 2-cell stage of 1-cell embryos from some mouse strains during in vitro culture is known as the 2-cell block. We investigated the specific factors involved in the 2-cell block of AKR embryos by means of a modified culture system, the production of reconstructed embryos by pronuclear exchange and a cross experiment. In a culture medium with phosphate, 94.6% of 1-cell embryos from the C57BL mouse strain developed to the blastocyst stage, but 95.7% of embryos from the AKR mouse strain showed 2-cell block. Phosphate-free culture medium rescued the 2-cell block of AKR embryos and accelerated the first cell cycle of the embryos. Co-culture with BRL cells and a BRL-conditioned medium fractionated below 30 kDa also rescued the 2-cell block of AKR embryos. Examinations of in vitro development of reconstructed embryos and of embryos from F1 females between AKR and C57BL strains clearly demonstrated that the AKR cytoplast caused the 2-cell block. In the backcrossed female progeny between (AKR × C57BL) F1 males and AKR females, about three-quarters of the embryos were of the 2-cell blocking phenotype and about one-quarter were of the non-blocking phenotype. These results suggest that two genes are responsible for the 2-cell block of AKR embryos.