Recombinant human granulocyte macrophage colony-stimulating factor (rhGM-CSF) was one of the first of the myeloid growth factors to become available for clinical trials. Phase I studies have demonstrated that the optimal administration is by continuous intravenous infusion or subcutaneous injections at doses of 4-5 micrograms/kg/day. Phase II trials in patients with a variety of malignancies who receive rhGM-CSF after standard doses of chemotherapy have demonstrated significant reductions of the duration of leucocytopenia. Use of rhGM-CSF after high-dose chemotherapy (with or without bone marrow rescue) suggest that this agent decreases the time to recovery of a normal blood count and reduces infective complications. Results in myelodysplasia and aplastic anemia have been less encouraging. The potential value of rhGM-CSF in the treatment of a variety of other conditions including AIDS and the leukemias is being tested and the early results are discussed.