This project proposes to apply the relatively underused methods of microhistory to track the trajectory of research on the causative agent of chicken sarcomas, what we now call the Rous Sarcoma virus (RSV) at Rockefeller University from 1911-1998. That a tumor could be caused by a virus—a filterable living disease agent—was first suggested by Peyton Rous but his ideas were met with much resistance. Although he quit working on RSV in 1915, research on different aspects of its nature and the mechanisms by which it induced tumors took on multiple incarnations at the Rockefeller for nearly a century culminating in the crystallization of specific protein responsible for the tumor inducing effects of the virus in the late 1990s. Support is requested from the CHSTM for research at multiple sites where relevant materials are archived.