The developmental potential of the inner cell mass (ICM) of the cultured mouse embryo was determined by testing the ability of the ICM to regenerate trophoblast in vitro. ICM's isolated by immunosurgery from either single or chimeric embryos were able to regenerate trophoblast when they were isolated at 69 hours of culture from the 2‐cell stage, but they had lost this capacity by 93 hours of culture. Trophoblast regeneration by isolated ICM's did not appear to require either a critical cell mass at the time of isolation or cell proliferation during regeneration.