Since Peyton Rous in 1911 discovered the agent capable of producing a myxosarcoma in chickens, much speculation has suggested that viral agents are the cause of cancer. At the present time a relationship between virus and tumor in man appears on the horizon,1 and an increasing number of agents has been proved capable of causing tumors and leukemia in small animals. The recently reported Moloney agent and the polyoma virus, which produces a variety of tumors in several small animals, have been discoveries of the 1960's.
Meanwhile the Rous agent has been continuously nurtured. It has been purified, grown in cell culture, and planted in chick embryos; methods of titrating its strength have been developed. Its general character is unchanged. It is capable of producing tumors in several species of birds; the tumors always develop from cells of mesenchymal origin. Today strains are recognized, with different biophysical and biological