Colony stimulating factors (CSF) for white blood cells are formed in the bone-marrow and regulate proliferation and differentiation of the myeloid cells. Several of these hormones are cloned and manufactured by a recombinant technique for clinical use. The granulocyte-macrophage-colony-stimulating factor (GM-CSF) and the granulocyte-colony-stimulating factor (G-CSF) both belong to this group. Trials are still at the commencing stage. In vitro investigations, animal experiments and human experiments have rendered promising results. After intensive chemotherapy followed by treatment with GM-CSF and G-CSF, it has been demonstrated that the period of neutropenia following bone-marrow suppression is abbreviated. Accelerated bone-marrow regeneration and reduced tendencies to infection have been demonstrated compared with so-called control patients. In patients with the myelodysplastic syndrome, marked increase in the neutrophile granulocytes in the peripheral blood has been observed during treatment with growth factors. Long-term treatment with G-CSF has proved particularly effective in chronic idiopathic and cyclic neutropenia. On the other hand, the growth factors can scarcely improve the prognosis in severe aplastic anaemia. Animal experiments and a single human trial suggest that G-CSF and GM-CSF treatment are indicated in bone-marrow insufficiency secondary to radioactive irradiation accidents. GM-CSF has normalized the leucocyte counts in AIDS patients with leucopenia without any marked alteration in the tendency to infection. Future studies will show whether simultaneous treatment with growth factors and anti-viral and anti-leukaemic treatments, respectively, can improve the therapeutic results.