1. The development of 6 strains of Ep-L cells from human blood was reported. In each instance the Ep-L cells appeared in cultures which had some connection with the administration of estrogenic materials, either to the donor of the blood or to the primary explant of blood in vitro. The possible role of estrogens was discussed, but no final conclusions were reached.
2. The development of stable human Ep-L cells did not require the participation of cells from leukemic blood.
3. Morphologic, karyotypic, and mitotic data on the Detroit blood cell strains were presented.
4. The morphogenesis of the Ep-L cells was described. The most likely source of the Ep-L cells in blood cultures appeared to be lymphocytes, but the role of monocytes could not be ruled out.
5. The significance of our findings with respect to transplantation studies was indicated.