Cells of the bone marrow are chiefly dedicated to two processes; the production of blood, and the production of bone that houses the hematopoietic organ. The majority of our understanding of these processes comes from data focused on one of these functions. Yet, in vivo the processes are intermixed. Our recent data demonstrate that human osteoblast-like cells have important accessory roles in hematopoiesis. These data include the demonstration that human osteoblast-like cells; support the growth of primitive human hematopoietic progenitors (CD34(+) cells) in short and long term cultures and, synthesize multiple cytokines believed to regulate hematopoiesis. Based upon anatomic and developmental findings characterizing hematopoietic cells in close approximation with endosteal cells, and these findings, we hypothesize that osteoblasts play a critical role in hematopoietic cells development in vivo.