Analysis of the events that regulate development of red blood cells or granulocytes has led to therapies altering clinical conditions associated with anemia or neutropenia. The development of therapeutic approaches to target conditions associated with lymphopenia, such as AIDS, has been thwarted by limited techniques for studying T–lymphocyte development. We describe an in vitro system in which human bone marrow CD34⁺ cells proliferate, acquire the expression of the lymphoid–specific RAG–2 gene and a broad repertoire of rearranged T–cell receptor genes, develop the ability to produce T cell–specific interleukin–2 and achieve a range of T–cell immunophenotypes. The cells also become susceptible to infection with the T–lymphotropic strain of human immunodeficiency virus–1, HIV–1_IIIB. This culture system induces human T lymphopoiesis and may permit further analysis of the events regulating human T–lineage differentiation. It provides a preclinical model for screening stem cell gene therapies directed toward AIDS.