Knowledge is not necessarily, indeed seldom, put to work in the way that the initial producers of that knowledge envisioned and the early history of research on cancer viruses at the Rockefeller University offers a good case study of this point. The idea that viruses might be involved in the causing cancers or tumours was proposed in 1911 by the Rockefeller University physiologist Peyton Rous when he found that a cell-free extract of tumour tissue was capable of transmitting chicken sarcoma to unaffected fowl. Arriving at the Rockefeller around the same time, James B. Murphy a young medical researcher joined Rous in studying the tumour problem and the two men published many papers reporting the results of their investigations. But the knowledge gained from same experiments resulted in diametrically opposite conclusions by the two men. Although Murphy agreed with Rous on the nature of the sarcoma - that it was a true malignancy - he disagreed on the issue of its cause. Rather than viewing the causative agent as an exogenous virus, Murphy considered it to be more in the nature of an "enzyme," or some other endogenous principle, i.e. from within the cell, highlighting the different thought styles within biology at the time. While Rous shifted his attention to blood physiology within a few years of his discovery and only returned to the problem of cancer viruses in the 1930s using the Shopes papilloma virus as his experimental subject, Murphy, who became the head of Cancer Laboratory at the Rockefeller, continued to investigate the nature of the sarcoma and its cause. By the 1930s he had concluded that the sarcoma was a result of a somatic mutation and that the RSV was in the nature of a "transmissible mutagen." Meanwhile others such as William E. Gye and Christopher Andrewes in England who had begun to work on sarcoma during the 1920s were more sympathetic to Rous's initial conception of the tumour agent as a virus. Murphy's contributions were thus instrumental in keeping open a dialogue through which tumour viruses survived as research objects despite skepticism, largely because of the ambiguity surrounding the nature of what a virus was during that period. In this paper, I examine the ways in which the knowledge about sarcomas gained by Murphy was put to work, not only in understanding bird sarcomas but also in getting a handle on their etiology.