Distinguished for his work on Murine leukaemia and the function of the thymus. In 1956, he showed that the thymus alone of normal tissues contained a soluble substance capable of stimulating lymphocytopoiesis. He provided evidence of the importance of this factor in the development of lymphatic leukaemia in mouse strains genetically susceptible to the disease, and has analysed the process of leukaemogenesis in the thymus in detail. His quantitative work on the size and cellular composition of the mouse thymus and thymic grafts has been essential to the modern development of concepts of the thymus as a major immunological organ.