Mouse ICM's that were isolated from blastocysts by immunosurgery and subsequently cultured for four to five days were tested for their ability to form trophoblast giant cells (TGC's) in vitro. The cores were isolated from such ICM's by a second immunosurgery and cultured for 7 to 14 days. After two days of culture a variable percentage (30%--90%) of the ICM cores produced outgrowths containing cells that resembled those of cultured ectoplacental cone. Then the outgrowths became amitotic and produced vascuolated cells that enlarged to dimensions of 200 X 375 micrometer with nuclei as large as 53 micrometer in diameter. The time course of their development from the ICM cores, their morphology and their nuclear diameters suggest that these large cells were TGC's.