To study the regulation of the early stages of hematopoiesis, cDNA representational difference analysis was used to isolate genes that were differentially expressed in primitive hematopoietic progenitors. The reasoning was that such genes were more likely to provide functions important to hematopoietic stem cells and progenitors. One of the genes identified through this approach encodes mouse Jagged2 (mJagged2). Using quantitative reverse transcription-polymerase chain reaction, it was shown that mJagged2 was differentially expressed in c-kit(+) hematopoietic progenitors, including those with the phenotypes of Lin(-) c-kit(+) Rh(lo) Ho(lo) and Lin(-) c-kit(+) Rh(hi) Ho(lo), and that they have been shown to be highly enriched for long-term and short-term repopulating hematopoietic stem cells, respectively. Western blot analyses showed that endothelial cells also expressed high levels of Jagged2, but stromal fibroblasts did not. Using a coculture system we found that exogenous, full-length mJagged2 promoted the survival and proliferation of hematopoietic progenitors, including the high-proliferative potential colony-forming cells. Direct cell-to-cell contact was required for this effect. Taken together, these findings indicate that both c-kit(+) hematopoietic progenitors and endothelial cells express Jagged2 and that exogenous, full-length Jagged2 promotes the survival and proliferation of hematopoietic progenitors.