Pronlonged in vitro growth of murine bone marrow has been achieved using a flask culture system. We report the adaption of the technique to the growth of human bone marrow. In this system, CFU-GM and CFU-E are maintained for 4–9 wk. Morphologically recognizable granulopoiesis occurs for 4–6 wk and erythropoiesis for 1.5–2.5 wk. Functional T lymphocytes are maintained for at least 5 wk. The adherent population is composed of macrophages, fibroblast-like cells, and flat pavement- like cells. Fat-containing cells are not prominant. This culture system provides a means to study human hematopoietic cell proliferation and differentiation in vitro, as well as to examine interactions between stromal cells and hematopoietic and lymphoid progenitors.