We disrupted the Mcl-1 locus in murine ES cells to determine the developmental roles of this Bcl-2 family member. Deletion of Mcl-1 resulted in peri-implantation embryonic lethality. Mcl-1(-/-) embryos do not implant in utero, but could be recovered at E3.5-4.0. Null blastocysts failed to hatch or attach in vitro, indicating a trophectoderm defect, although the inner cell mass could grow in culture. Of note, Mcl-1(-/-) blastocysts showed no evidence of increased apoptosis, but exhibited a delay in maturation beyond the precompaction stage. This model indicates that Mcl-1 is essential for preimplantation development and implantation, and suggests that it has a function beyond regulating apoptosis.