The establishment of a cell culture system for the clonal development of haemopoietic cells has made it possible to identify the proteins that control growth and differentiation of different haemopoietic cell lineages and to discover the molecular basis of normal and abnormal cell development in blood-forming tissues. A model system with myeloid cells has shown that normal haemopoietic cells require different proteins to induce cell multiplication and cell differentiation, and that a cascade of interactions between proteins determines the correct balance between immature and mature cells in normal development. Gene cloning has shown that there is a family of different genes for these proteins. Normal protein regulators of haemopoiesis can control the abnormal growth of certain types of leukaemic cells and suppress malignancy by inducing differentiation to mature, non-dividing cells. Genetic abnormalities that give rise to malignancy in these leukaemic cells can be bypassed and their effects nullified by inducing differentiation, which stops cells from multiplying. These haemopoietic regulatory proteins are active in culture and in vivo and have been used clinically to correct defects in blood cell development. The results provide new approaches to therapy.