Inner cell masses (ICMs) isolated immunosurgically from mouse blastocysts segregating the homozygous lethal mutants t0/t0 and tW5/tW5 were cultured in vitro. Presumed t0/t0 ICMs fail to grow after three days in culture (equivalent gestational day 7.5) when they consist of an outer layer of endoderm cells surrounding about 30 epiblast cells. Presumed homozygous tW5/tW5 ICMs develop to a more advanced stage in culture and on the seventh day (equivalent gestational day 11.5) consist of an inner core of disorganized ectoderm cells with a small proamniotic cavity, surrounded by multiple layers of endoderm cells.