The overall objective of this study is to determine the role of cellular immunity in fibroma tumor regression either as an anti-tumor cell function or an anti-virus function. Lymphocyte and macrophage cytotoxicity to fibroma tumor cells will be evaluated in adult rabbits with regressing tumors and newborn rabbits with progressive growing tumors (51Cr release test) to determine if newborn tumor-bearing rabbits have a defective cell-mediated immunity. Anti-tumor cell immunitiy of macrophages will be related to anti-viral immunity (ability to restrict virus replication; interferon production) in the adult and newborn rabbits. Lymphocyte function in adult and newborn rabbits infected with virus will also be evaluated in the in vitro 3HTdR lymphocyte stimulation test.