By transplanting human hematopoietic stem and progenitor cells in newborn immunodeficient mice, researchers have recently achieved development of a human adaptive immune system, consisting of B cells, T cells, dendritic cells, and some cells of the myeloid lineage, which together form respective structured lymphoid organs as thymus, spleen, and lymph nodes, and produce some functional immune responses. These will provide valuable models to analyze the impact of chemicals, pharmaceuticals, or species-specific infections on human hematolymphoid cells in vivo. This unit describes methods for isolation of the progenitor human cells and their injection into mouse to constitute an adaptive immune system.