Stromal cells of the bone marrow can provide the growth-promoting and differentiation-inducing molecules which are necessary for haemopoiesis. While the nature of these stimuli is largely unknown, the development of haemopoietic cells in association with stromal cells requires intimate cell contact. Molecules of the extracellular matrix, such as heparan sulphate, are able to bind growth factors and in this way the stromal cells may form microenvironmental niches which preferentially promote development of multipotent and committed cells along discrete lineages. Cells from some patients with acute and chronic myeloid and lymphoid leukaemias are defective in their ability to interact with stromal cells and consequently cannot survive in stromal cell-mediated long-term marrow cultures. We have exploited this phenomenon to obtain normal haemopoietic cells from patients with leukaemia, and to use these cells for successful autografting in patients with acute and chronic myeloid leukaemias.