Successive steps in T lymphocyte differentiation and T potential of human stem cells (HSC) can be tested in the following models: (a) the infusion of cells in NOD-SCID mice, (b) the injection of cells in renconstituted SCID/hu mice, (c) the differentiation of cells in fetal thymus organ culture (FTOC), and (d) on thymic stromal layers. Using mixed human-murine FTOC, we showed (a) TCR alpha beta, TCR gamma delta lymphocytes, NK cells, and dendritic cells complete their differentiation, (b) IL-7R alpha signaling and IL-7 are essential, (c) a detailed phenotypic and functional analysis of discrete successive steps of positively selected thymocytes, (d) an efficient transduction of genes in HSC with persistent gene expression throughout the T-lymphocyte differentiation, and (e) adaptation to submerging high oxygen culture increases the test sensitivity to a clonal assay. Other approaches are the in vivo SCID/hu reconstitution model. With this method small fragments of human fetal liver and thymus are implanted under the kidney capsule of an adult SCID mouse with result in an impressive human thymus organ, six months after transplantation. We use this model to study thymus T-cell developmental kinetics, development of gene-marked precursor cells and thymic homing of precursor cells.