Myeloid cell kinetics in mice treated with pure hematopoietic growth factors have been investigated using tritiated thymidine labeling and autoradiography. Mice were injected subcutaneously with 125 micrograms/kg granulocyte colony-stimulating factor (G-CSF) (in some cases 5 micrograms/kg), or 10 micrograms/kg of granulocyte-macrophage CSF (GM-CSF), or interleukin-3 (IL-3) every 12 hours for 84 hours. 3HTdR labeling was performed in vivo after 3 days of treatment. G-CSF increased the peripheral neutrophil count 14-fold and increased the proportion and proliferation rate of neutrophilic cells in the marrow, suppressing erythropoiesis at the same time. Newly produced mature cells were released into the circulation within 24 hours of labeling, compared with a normal appearance time of about 96 hours. By contrast, GM-CSF and IL-3 had little effect on either marrow cell kinetics or on the rate of release of mature cells, although GM-CSF did stimulate a 50% increase in peripheral neutrophils. Monocyte production was also increased about eightfold by G-CSF and 1.5-fold by GM-CSF, but their peak release was only slightly accelerated. While the peripheral half-lives of the neutrophilic granulocytes were normal, those of the monocytes were dramatically reduced, perhaps due to sequestration in the tissues for functional purposes. The stimulated monocyte production in the case of G-CSF required an additional five cell cycles, a level that might have repercussions on the progenitor compartments.