Granulocyte-macrophage colony stimulating factor (GM-CSF) and interleukin 3 (IL-3) target cells have been studied in vivo in subjects with normal hemopoiesis. GM-CSF administration elicits a rapid and sustained neutrophilia, monocytosis and eosinophilia due to a direct proliferative stimulus on all progenitors and precursors of the granulomonopoietic lineage. GM-CSF is also a powerful stimulator of erythroid burst forming unit (BFU-E) and megakaryocyte colony forming unit (CFU-MK) proliferation. Its action, however, does not extend to more mature erythroid and megakaryocyte cells suggesting the need for combined treatment with lineage-specific growth factors such as erythropoietin (Epo) or IL-6 to obtain a complete myeloid stimulation. When GM-CSF is discontinued, its action rapidly vanishes, and a rapid decline in the proliferative rate of target cells to values below the initial ones occurs. The potential clinical usefulness of this phenomenon in regard to cancer chemotherapy is discussed. IL-3 treatment induces only a rapid and marked eosinophilia. Chronic IL-3 administration, however, increases the proliferation of all myelopoietic progenitors and primes CFU-GM to become more sensitive in vitro to the action of granulocyte CSF (G-CSF), GM-CSF and IL-5. Whereas an increased IL-5 sensitivity seems devoid of therapeutic potential, the priming of G-CSF and GM-CSF action suggests rational scheduling for a combined treatment of IL-3 with other hemopoietic growth factors.