In humans, the role of bone marrow (BM) adipocytes in supporting haematopoiesis has been questioned. A co-culture system of CD34(+) cells seeded onto either BM undifferentiated mesenchymal stem cells or differentiated adipocytes showed that BM adipocytes did not support the maintenance of immature progenitors but enabled their complete differentiation along the myeloid and lymphoid pathways. These properties appear to be opposite to those of osteoblasts, although both cell types share a common mesenchymal progenitor. These results suggest that stromal cells play a variety of roles in the haematopoietic microenvironment, which could be significant in situations such as osteoporosis or ageing.