Haematopoietic growth factors play a very important role in the proliferation and maturation of medullary cells. To date, about fifteen of these factors have been characterized and are known to act either on the most immature progenitors, or on the proliferation compartment, or on already mature cells. In fact, myeloid lines depend on complex interactions between these various inductors. All these inductors are made of glycoproteins, and many of them are continuously located on the long arm of chromosome 5, but they do not represent a family of genes. Their receptors are not limited to normal haematopoietic cells, and this lack of selectivity accounts for certain side-effects observed in clinical use and for the (theoretical) risk associated with their use in the treatment of some malignant blood diseases or even solid tumours.