The present invention relates to methods, kits and compositions for expansion of hematopoietic stem/progenitor cells and providing hematopoietic function to human patients in need thereof. In one aspect, it relates to kits and compositions comprising a Notch agonist and an aryl hydrocarbon receptor antagonist. Also provided herein are methods for expanding the hematopoietic stem/progenitor cells using kits and compositions comprising a Notch agonist and an aryl hydrocarbon receptor antagonist. The hematopoietic stem/progenitor cells expanded using the disclosed kits, compositions and methods include human umbilical cord blood stem/progenitor cells, placental cord blood stem/progenitor cells and peripheral blood stem cells. The present invention also relates to administering hematopoietic stem/progenitor cells expanded using a combination of a Notch agonist and an aryl hydrocarbon receptor antagonist to a patient for short-term and/or long-term in vivo repopulation benefits.