[Antitumor drug screening at the National Cancer Institute, U.S.A].

The development of suitable screening systems is one of the most important determinants for the successful discovery of new anti-tumor drugs. The National Cancer Institute, U.S.A. has been playing a major role in the discovery and development of new agents by using various animal tumor models since 1955. In this article, the historical overview of this effort, and the difficulties and problems of this approach was reviewed. Also the new "Disease-Oriented Screening System", which is now under development as a new screening system at the National Cancer Institute was also described.