SUMMARY: Significant advancements have been made in the last 2 years concerning the role of the immune response in virus-induced feline tumors and the natural history of the feline oncornaviruses (fov). Observations of apparent horizontal transmission have been made in both feral and laboratory cat populations. Salivary excretion and intranasal infection have been demonstrated with the feline leukemia virus (FeLV). It has been shown that cats are not immunologically tolerant to the principal virion antigens as was once thought. Thymic aplasia accompanied by a depression of the cell-mediated immune response has been observed in cats incubating leukemia. New tumor cell membrane antigens have been demonstrated in cells exposed to these viruses. These antigens are also expressed in animals exposed to live virus. High antibody levels to the antigen are associated with protection against the progressive growth of virus-induced sarcomas, and antibody either is absent or present in lower quantities in animals with progressing virus-induced sarcomas. Approximately 13% of 71 healthy adult cats examined in the Boston area had antibody titers to the membrane antigen of 8 or higher. Our results and results of others reviewed here indicate that horizontal transmission should be considered as a significant possibility for the distribution of oncornaviruses among cats under natural conditions.