Immunodeficient mice reconstituted with human haematopoietic cells (termed humanized mice) are a powerful tool for studying human immune function in vivo. However, human innate immune cell development is not supported by current humanized mouse strains. Two new strains of humanized mice, termed MITRG and MISTRG, have been developed that support human monocyte, macrophage and natural killer (NK) cell development. A knock-in gene replacement strategy was used to replace genes encoding cytokines that are important for myeloid cell development — M-CSF, IL-3–GM-CSF and thrombopoietin — with their human counterparts in immunodeficient Rag2−/−Il2rg−/− mice (MITRG mice). MISTRG mice also carry a transgene encoding human SIRPα, which is thought to have a role in NK cell survival. The human monocytes and macrophages were fully functional in MITRG and MISTRG mice, and trans-presentation of IL-15 by these cells supported the development of functional human NK cells in MISTRG mice. Furthermore, immunosuppressive macrophages infiltrated and supported the growth of engrafted tumours in both mouse strains, similar to what has been observed in human tumours.