We have investigated the effects of thermal injury upon myelopoiesis. IL-3, GM-CSF, and IL-5 were used to stimulate myeloid colony formation. IL-3 induces early myeloid progenitors and a more developed myeloid progenitor, the granulocyte-macrophage colony-forming unit (GM-CFU), to multiply and develop into mature myeloid cells. GM-CSF induces GM-CFU to become mature myeloid cells, while IL-5 induces eosinophil progenitors to become mature eosinophils. Stem Cell Factor (SCF) + IL-6 and FLT3 ligand, which have no effect on colony formation by themselves, were used to enhance the effects of IL-3 and GM-CSF, respectively. We found that thermal injury increased the number of early myeloid progenitors and GM-CFU in the spleen with either IL-3 or GM-CSF as a stimulant. Thermal injury increased the number of early myeloid progenitors in the bone marrow when GM-CSF, but not IL-3, was used to stimulate colony growth. Also, thermal injury increased the numbers of eosinophil progenitors in rat spleen and bone marrow and increased splenic levels of IL-5 mRNA.