Abstract: Two novel populations of human short term repopulating cells are described. In particular, The inventors have shown that sublethally irradiated NOD/SCID-b2M−/− mice allow the efficient engraftment of two previously undescribed populations of human short term repopulating cells (STRC) that do not produce detectable progeny in the more widely used NOD/SCID mouse. These novel cells are designated short term repopulating cells—myeloid (STRC-M) and short term repopulating cells—lympho-myeloid (STRC-ML) to reflect their different lineage potentials. The invention includes an assay for detecting STRC-M and STRC-ML which is useful in a wide range of applications including assessing of the engraftment potential of human hematopoietic cells, testing the toxicity of drugs on hematopoietic cells and in assessing the viability of hematopoietic cells that have been stored and processed.