A therapeutic trial of interleukin-3 (IL-3) was carried out in four patients with aplastic anemia refractory to the prior therapies. Daily subcutaneous doses of 2.5, 5.0 or 7.5 micrograms/kg was given for 7 or 14 days. In a patient who had co- and immediate boost-administration of granulocyte colony-stimulating factor (G-CSF) and/or erythropoietin (Epo) and another who had sequential administration of G-CSF and Epo two weeks after IL-3, definite hematological response was obtained during the course after IL-3. In one patient, moderate to severe side effects consisting of facial edema, conjunctival bleeding, chills and fever, were observed after two days' administration of IL-3. Co- or sequential administration of other hemopoietic factor(s) may be essential in IL-3 therapy for aplastic anemia.