United States government. Defense Nuclear firms, Hoffman-LaRoche and Ciba-Geigy, and by Agency, Armed Forces Radiobiology Research In- the American firm, Travenol. stitute. SIEGMUNDJ. BAUM Furthermore, we greatly appreciate the gener- G. DAVID LEDNEY ous financial support by the Swiss pharmaceutical vi Xlll List of Contributors Part I Regulation of Stem Cell Proliferation L. G. Lajtha 1 Regulation of Stem Cell Proliferation 3 L. G. Lajtha and E. G. Wright Introduction 3 Experimental Evidence 3 Summary 7 References 7 Contents 2 Surface Antigens of Hemopoietic Stem Cells: The Expression of BAS, Thy-1, and H-2 Antigens on CFU-s 9 Ger van den Engh, Jim Russell, and Diane de Cicco Introduction 9 Materials and Methods 10 Results 11 Discussion 13 Summary 14 Acknowledgments 14 References 15 3 The Regulation of Hemopoiesis: Effect of Thymosin or Thymocytes in a Diffusion Chamber 17 S. J. Sharkis, A. Ahmed, L. L. Sensenbrenner, W. W. Jedrzejczak, A. L. Goldstein, and K. W. Sell Introduction 17 Materials and Methods 17 Results 18 Discussion 20 Summary 21 Acknowledgments 21 References 21 4 Anti-CFU-s Activity of Rabbit Anti- mouse Brain Serum: Mechanism of Action 23 Solomon S. Adler, Richard D. Kuznetsky, and Frank E. Trobaugh, Jr. Introduction 23 Materials and Methods 23 Results 26 Discussion 29 Summary 31 Acknowledgments 31 References 31 vii Contents Part II Ralph van Furth, Theo J. L. M. Goud, and Physiology of Committed Dick van Waarde Stem Cells 33 Introduction 65 D.