- The Structure of the Thymus - Thymus Grafts - Lymphocyte Migration from the Thymus - Regulation of Thymic Lymphopoiesis - Effect of the Thymus on Peripheral Lymphoid Organs - The Thymus and Immune Responses - Trophic and other Functions of the Thymus - The Thymus and Lymphoid Leukaemia - The Thymus and Carcinogenesis - General Discussion / Abstract : In the mouse and rat, the thymus is of critical importance in determining susceptibility to lymphoid leukaemia. In fact the studies on this relationship by McEndy, Boon and Furth (1944) provided the first definitive evidence that the thymus has any functional activity at all in the body. Although the role of the thymus in lymphoid leukaemia development is still not fully understood much has been learned of the basic mechanisms involved. The sudden burst of interest in the thymus during the last five years more than justifies the appearance of a monograph on those aspects of its function which bear particularly on immunity and the processes associated with the development of malignant disease. Of those who are actively working in the field I believe that Dr. DONALD METCALF is uniquely suited to produce such a work. His demonstration of a lymphocytosis stimulating factor from thymic tissue in 1956 was almost the first evidence of physiological activity by the thymus. He has played an important part in the analysis of the leukaemogenic process in mice and his recent work on thymic grafting has been almost disconcertingly productive of new ideas. I feel confident that this monograph will be of great value to a wide range of investigators and teachers interested in obtaining a clear summary of the present posi- tion. Incidentally, Dr. METCALF has provided a well chosen sample of factual material from which others who may wish to question some of his speculations, can draw their own conclusions. He has been careful to keep the observational and experimental facts clearly distinct from theoretical matters but has not shirked from making his con- tribution to the current ferment of immunological theory.