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T cell precursors, which are derived from foetal liver and later from bone marrow, are obliged to colonize the thymus to differentiate into mature T cells. Multiple factors, secreted by cells of different types and origins, are involved in the constitution of a suitable micro-environment. Thymic epithelial cells have an endo-ectodermal origin, while the epithelial rudiment is secondarily colonized by mesodermal precursors of lymphocytes, macrophages and interdigitating cells. The migration of stem cells into the thymus is controlled by chemotactic factors which are derived from epithelial cells as well as from mature thymic medullary cells. Intra-thymic differentiation is controlled by thymic humoral factors secreted by epithelial cells. One of these factors, the "facteur thymique serique" (FTS) (now called thymulin), has been isolated from the serum and also plays a role in the later extra-thymic maturation process. Thymic hormones are able to generate differentiation antigens on the surface of pre-thymic precursors. They have complex effects on the immunological function of post-thymic precursors. In addition to thymic hormones, other signals, such as interleukins, are necessary. Dendritic-shaped phagocytic cells of the thymic reticulum are able to secrete IL1 which is necessary for the secretion of IL2 by mature thymic medullary lymphocytes. MHC antigens on thymic epithelial cells and other factors, such as prostaglandins, also play important roles in intra-thymic T cell differentiation.