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Mouse blastocysts in culture have been treated with increasing concentrations of cytosine arabinoside, bromodeoxyuridine or Colcemid. Concentrations of all three antimetabolites have been found which interfere with neither hatching of the blastocysts from their zona pellucidae nor subsequent attachment of the blastocysts to the culture dish, but which eventually result in death of the inner cell mass (ICM) and its derivatives. The effect upon the ICM is selective at these antimetabolite concentrations since many or, in some cases, all trophoblast cells continue to survive, and by a number of criteria, undergo normal patterns of differentiation and development.