The A.A. have investigated the possibility of survival and growth of neoplastic cells after hetero-transplantation into immunologically incompetent hosts. Tumors were: mouse Sarcoma 180 (transplanted into cat foetuses and new-borns) and Lymphosarcoma (transplanted into rat foetuses). All animals showed neoplastic growth. These preliminary studies have proved that the transplant of a murine tumor into animals of different species, like the cat or the rat, is possible provided that the host is injected during its foetal life or during the very first days after birth.