Progress in our understanding of the hematopoietic system as well as novel cellular and molecular biology techniques are increasingly promoting the ex vivo manipulation and therapeutic use of hematopoietic stem and progenitor cells. For both, development of stem cell therapies and basic hematopoietic research, test systems for hematopoietic stem cells are required to monitor the intrinsic and ex vivo‐induced properties of these cells.

In vitro assays for primitive hematopoietic cells (colony‐forming units‐blast, cobblestone area‐forming cells, long‐term culture‐initiating cells [LTC‐IC]) have been established which demonstrate the proliferative and differentiation capacities of these populations. The potentials of these assays have been recently enhanced by the extended LTC and the switch LTC modifications. Although some hematopoietic cells characterized in vitro have the multipotential and proliferative properties of pluripotent hematopoietic stem cells (PHSC), their capacity to long‐term repopulate hematopoiesis in vivo, a hallmark of PHSC, has not been established. Without this confirmation, populations defined in vitro should not be considered the equivalent of PHSC.

In animals, the properties of primitive hematopoietic cells can be systematically analyzed by multiple in vivo assays. Therefore, various strategies have been pursued to develop an animal model for human hematopoiesis. In fetal sheep and immunodeficient mice, the functions of human PHSC are reproduced, and long‐term multilineage repopulation capacity and extensive proliferative potential have been demonstrated for xenografted human cells. Thus, both models can be considered stem cell assays and may significantly enhance the study of early hematopoiesis and the development of therapeutic strategies. Stem Cells 1997; 15(suppl 1): 185‐195