This protocol details the steps for isolating human CD34+ cells from human fetal liver. It also explains how to prepare human fetal thymus for immediate use or for freezing, as well as the process for thawing. The CD34+ cells are hematopoietic progenitor cells and can be used to generate humanized mice through reconstitution of immune cells via IV injection after bone marrow ablation. These cells can also be used for mixed lymphocyte reaction experiments.