Summary.-Five out of 18 primary explants of human carcinomata obtained at operation have been grown progressively for a minimum of one month in thymectomized, x-irradiated mice reconstituted with syngeneic bone marrow and subsequently treated also with anti-thymocyte serum. All the tumours which proliferated were of gastrointestinal origin, growing locally but not metastasizing although direct invasion of the ribs occurred in one case. No implanted breast carcinomata grew in this system. THE aim of the present experiments was to determine whether immunedeficient mice provided a suitable environment for the progressive growth of implanted human carcinomata.Numerous methods of heterotransplantation have been described and reviewed (Chesterman, 1959). It is generally accepted that cell mediated immunity is largely responsible for xenograft rejection and that anti-thymocyte serum (James, 1967) and x-irradiation (Toolan, 1955) are effective immunosuppressants in this context. Thymectomy appears to enhance the action of anti-thymocyte serum (ATS) on first generation tumour graft survival in mice (Jeejeebhoy, 1967;Phillips and Gazet, 1970). Thymectomy combined with irradiation and bone marrow reconstitution to produce immune deprived mice is now a well known immunosuppressive regimen. A relatively more permanent immune impairment is produced in deprived mice than with ATS treatment alone, which is specific for T lymphocyte depletion and dependent on serum potency and the period of its administration. In these bone marrow reconstituted mice, a lymphoid population of cells will eventually develop so that ATS administration would be an added precaution against the cell mediated response returning. In order to keep ATS toxicity to a minimum, low doses of serum were injected twice weekly until the graft seemed to have established.
MATERIALS AND METHODSMale and female adult A2G mice, inbred for at least 45 generations, were used. The mice were weaned and separated according to sex at 3 weeks of age. Female New Zealand white rabbits weighing between 2-5 and 3-5 kg were employed for ATS preparation.Two ml of a thymocyte suspension prepared from the thymuses of 8 A2G mice were injected intravenously through the ear vein of each rabbit according to the method of Levey and Medawar (1966