The use of semi‐solid cultures of bone marrow cells is briefly reviewed. In the last two decades this technique has greatly improved our understanding of the hierarchical structure of cell production and its regulatory factors. It is now well established that relatively few multipotent stem cells, with self‐renewing capacity, are feeding cells into the differentiating pool of all the different lineages of mature cells found in blood. The proliferation and differentiation in this system are controlled by specific haemopoeitic growth factors, which also regulate the function of the mature cells. Several of these factors are now produced in recombinant form, allowing further studies of their biological significance. Clinical trials with two of the factors have supported the experimental evidence for theirin vivofunction, and they can be administered without serious side effects. The clinical access to these factors is promising for future treatment of cancer patients and may possibly completely change the outcome of several haematological and infectious disea