C-type RNA viruses of avian, feline and simian origin induce sarcomas in marmosets and a simian herpesvirus induces lymphomas and/or lymphocytic leukemias. Virus genome expression is usually repressed to various degrees in vivo, but becomes derepressed if the tumor cells are grown in vitro. The high susceptibility of marmosets to oncogenic viruses and the hematopoietic chimerism between offspring make them ideal animals for studying not only the oncogenic activity of animal tumor viruses and of potentially oncogenic materials but also the immune response to the viral-induced neoplastic diseases.