Hemopoiesis was analyzed in a miniaturized long-term culture of human bone marrow cells by quantifying the production of granulocyte-macrophage progenitor cells. As in the conventional long-term culture system, hemopoiesis was dependent on the presence of a marrow-derived adherent layer. Adipocytes proved to be essential for long-term proliferation. Optimal growth conditions were maintained by incubation in McCoy's medium supplemented with hydrocortisone, fetal calf serum, and horse serum. When calculated back to the volume of conventional cultures, the numbers and kinetics of nucleated cells and granulocyte-macrophage colony-forming cells were comparable in both culture systems. The microsystem is therefore suitable for performing multiple analyses on small samples of cells.