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Immunosuppressed mice have been used to support the growth of xenogeneic human and animal malignant cell populations. The optimal conditions for tumor growth are neonatal thymectomy coupled with antithymocyte serum or thymectomy, followed by whole-body irradiation and bone marrow reconstitution. When mice are inoculated with a mixture of normal and malignant cells, the malignant cells have a selective advantage. No such selectivity is found when the mixed populations are grown in vitro. Human tumors may also be grown in immunosuppressed mice. These tumors retain the organization of the original tumor in the human host. The advantages of this system to cancer researchers are discussed.

CBA mice rendered immunodeficient by thymectomy, potentially lethal gamma-irradiation and reconstitution with bone marrow cells were used to grow a wide variety of human tumours as subcutaneous implants. Samples of human melanoma obtained at surgery were successfully passaged by transplantation and produced rapidly growing tumours, some of which metastasized to lung, lymph nodes and the para-aortic node; this system was used as a model for the study of immunotherapy of melanoma. Preliminary results show that intratumour injections of C. parvum retard or inhibit the growth of melanoma transplants and, therefore, do not require the involvement of T lymphocytes, whilst BCG has no effect on growth rate.