INTERNATIONAL COLLABORATIVE ACTIVITIES of the Division of Cancer Treatment, National Cancer Institute, U.S. A., have a long-standing history, and in recent years have expanded to such an extent that an Office of International Treatment Research has been created. The scope of these activities encompasses three areas: 1) participation in International Collaborative Agreements with official national research entities. 2) direct NCI-supported projects, and 3) miscellaneous, less formalized programs. These are described and specific recent contributions are summarized. Emphasis is placed on the importance of international collaboration and exchange of information in searching for new drugs and in maximizing progress in cancer treatment.