Acute myelomonocytic leukemia develops in 10-30% of irradiated (300 rad) SJL/J mice, after a lag period of around one year. Additional treatment with dexamethasone shortly after irradiation increased leukemia incidence up to 50%. Experiments were conducted in order to demonstrate the existence of preleukemic cells in irradiated mice and to explore the possible role of dexamethasone, cyclophosphamide, and different hemopoietic growth factors on their promotion to overt leukemia. Transplantation of bone marrow cells from mice exposed to 300 rad plus dexamethasone into appropriate recipients, performed 4-5 months after leukemogenic treatment, resulted in acute myeloid leukemia (AML) development of donor origin in 70% of the recipients. Transfer of fractionated preleukemic bone marrow showed that the highest AML incidence developed in the recipients of fractions enriched in early hemopoietic precursors. The promoting effect of dexamethasone on preleukemic cells was confirmed by demonstrating its similar coleukemogenic effect whether administered within several hours or 130 days after radiation. Treatment with cyclophosphamide shortly after radiation could not replace the dexamethasone effect but was found to be complementary to the coleukemogenic effect of dexamethasone. Early administration of hemopoietic growth factors (starting 14 days after radiation and dexamethasone) showed that colony-stimulating factor (CSF) 1 increased the AML incidence (75%) and reduced its latency. Treatment with recombinant granulocyte-CSF (rG-CSF) had a reduced effect and recombinant granulocyte-macrophage CSF (rGM-CSF) had no promoting effect. However, administration of different factors several months after the leukemogenic treatment revealed that rGM-CSF increased AML incidence (75%) and shortened its latency, whereas rG-CSF and CSF-1 had no effect. In contrast, the late administration of recombinant interleukin 6 reduced AML incidence significantly (23%). The present results indicate that murine radiation induced AML is a multiphase process involving radiation induced preleukemia that can be promoted by different treatments.