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1. Introduction Animal models with a human hematopoietic and immune system are now important tools for studying the human immune system as well as human-specific infections, such as HIV-1 [1, [...], Immunodeficient mice are becoming invaluable tools in human stem cell and tumor research. In this study, we generated Rag-2/Jak3 double-deficient ([Rag.-2.sup.-/-][Jak3.sup.-/-]) mice with a C57/BL6 and Balb/c genetic background and compared the human lymphohematopoietic cell engraftment rate. Human cord blood-derived [CD34.sup.+] hematopoietic stem cells were successfully engrafted into Balb/c [Rag-2.sup.-/-] [Jak3.sup.-/-] mice; however, the engraftment rate was far lower in C57/BL6 [Rag-2.sup.-/-][Jak3.sup.-/-] mice. Transplantation of human peripheral blood mononuclear cells resulted in the same tendency. Thus, a Balb/c background offers superior engraftment capacity than a C57/BL6 background and provides an attractive model for human hematopoietic cell engraftment.