The Third Annual Symposium on Health Sciences attracted over 400 scientists from 15 countries. It was held at the National Academy of Sciences in Washington, D.C. The theme of this meeting was "Thymic Hormones and Lymphokines: Basic Chem- istry and Clinical Applications." The meeting emphasized the state of the art with regard to chemistry, mech- anism of action, and clinical applications of thymic hormones and lymphokines. In addition to the five sessions, the chairmen of each session were asked to prepare a synthesis and overview of each of the sessions. Some of the chairmen used this time to summarize the new material presented while others addressed important areas of conflict and terminology. The chairmen of the plenary sessions prepared short summary papers which have been incorporated into this volume at the end of each major section. We also invited several of the scientists who took part in the poster presentations to submit short papers. In addition to the scientific sessions, there were two awards and lectures. One, the Abraham White Distinguished Science Award, was given to Dr. Robert C. Gallo, who gave the keynote lecture. He was recognized for his pioneering research and scientific contributions to our understanding of the regulation of hematopoietic cell differentiation and the role of viruses in the development of human leukemias.