Bone marrow-derived adherent cell layers, containing endothelial cells, fat cells and macrophages will support prolonged haemopoiesis in vitro. Evidence suggests that the adherent layer is acting as an in vitro haemopoietic inductive environment for stem cell proliferation and differentiation into the variety of committed progenitor cells of the granulocytic, erythroid, megakaryocytic and lymphoid lineages. Using this system we have analysed the factors controlling proliferation of stem cells (CFU-S), differentiation and maturation of granulocyte/macrophage precursor cells (CFU-C), leukaemic transformation by chemicals and viruses, the role of environment versus stem cells in the aetiology of haemopoietic aplasias and the possible usefulness of long-term cultures for bone marrow transplantation.