It has been shown previously that the intact blastocyst of the mouse can regulate tumor formation and colony formation of murine embryonal carcinoma. This effect is consistent with the close histogenetic correspondence between embryonal carcinoma and the inner cell mass of the blastocyst. The ability of inner cell mass, blastocele fluid, and inner and outer surfaces of trophectoderm to abrogate colony formation of a variety of malignant tumors has now been tested. Direct contact of the embryonal carcinoma cells with the blastocele surface of trophectoderm proved to be necessary for abrogation of colony formation of embryonal carcinoma. This effect was not seen with any of the other tumors tested. Some tumors, which lack a normal cellular counterpart in the blastocyst, grew poorly in the blastocele unless a fistula was made in the wall of the blastocyst. Colony formation of the embryonal carcinoma was regulated in blastocysts with fistulas, but the other tumors were not regulated under these conditions. It is concluded that colony formation of embryonal carcinoma cells is regulated by direct contact with the trophectoderm of its corresponding embryonic field in an unknown but specific manner.