Fetal thymus organ cultures support a full range of T-cell precursor differentiation in vitro, including TCR gene rearrangement and expression. This provides an accessible model system in which the intra-thymic regulation of T-cell development can be investigated. Thymus organ cultures can be manipulated by adding antibodies to block different cell surface components on stromal or lymphoid elements. In addition, it is possible to deplete thymus lobes of their lymphoid cells and recolonize them with T-cell precursors of a different MHC haplotype to produce chimeric lobes. Recolonization can also be achieved with defined numbers or types of precursor cells, including a single micro-manipulated cell. These approaches have been used to obtain information on the signals regulating intra-thymic proliferation, T-cell lineage relationships, antigen receptor diversification within the thymus and the cellular interactions and intracellular mechanisms regulating selection of the antigen receptor repertoire.