AbstractMarsupials are useful for cancer research since they are born in a fetal stage of development with little immunologic competence. In this study, mouse melanoma xenografts were grown in neonatal gray opossums (Monodelphis domestical injected at 12-32 days of age. Tumors appeared between 1 and 3 weeks postinjection and at least 73% of tumor-bearing animals rejected their tumors. Initiation of tumor growth was rare in animals over 24 days of age and, in most cases, tumors size peaked at about 50 days of age after which tumors quickly regressed. These results are discussed with respect to the development of homeothermy (25 days of age) and gain of postpartum independence (50 days of age) in this marsupial species.