Viral induction of leukemia and solid tumors in the chicken were reported in the first decade of the 20th century by Ellerman and Bang (1908, 1909) in Denmark and Rous (1911) in the United States. Over the next 50 years, a small group of workers studied a variety of virus-induced neoplasms in the chicken and developed a number of sarcoma and acute leukemia virus strains (Beard 1963). In addition, the viral etiology of lymphoid leukosis, a naturally occurring B cell lymphoma of mature chickens, was established (Burmester 1947). However, it was not until the 1960s that research on viral induction of neoplasms in mammals and man was seriously undertaken (Gross 1961). Thus, the background information on the biology of oncogenic viruses and the neoplasms they induced in the chicken was far ahead of such information in other species. By 1960, cell culture assays for neoplastic transformation by the avian sarcoma viruses (ASV) and for infection by the closely related lymphoid leukosis viruses (LLV) that do not transform cells in culture had been developed (Rubin 1960, Temin and Rubin 1958), opening the way for intensive biological studies of these viruses. In 1964, Temin suggested that the RNA viruses, ASV and LLV, reproduced through a DNA intermediate that integrated into the host genome. This suggestion explained the persistence of ASV genetic information in transformed