Interleukin 1 alpha (IL-1) is a polypeptide/glycoprotein growth factor with multiple functions including the modulation of hematopoietic cell proliferation and differentiation. In vivo studies were performed with C57BL/6J mice injected with 0, 0.2, or 2.0 micrograms of IL-1 24 hr before or after lethal total body irradiation (TBI) (9.5 Gy). More mice in the groups administered IL-1 before TBI survived (90% of the 2.0 micrograms group) than those treated 2 or 24 hr after TBI, which was still slightly superior to the uninjected group, which all died within 15 days (p = .0001). Proliferation of bone marrow granulocyte/macrophage colonies following split dose TBI was also greatest for mouse groups treated with IL-1 prior to TBI. These experiments support data from other investigators that IL-1 stimulation of BM is related to IL-1 timing with respect to TBI. Stimulation of hemopoiesis was also assessed in terms of changes in peripheral blood and BM cell numbers and cell cycle kinetics using an electronic particle counter and flow cytometric techniques. Mice injected with 2 micrograms of IL-1 showed an initial decline (at 3-6 hr) and then a selective proliferation (24-48 hr) of early and more committed progenitor cells to 125% and 200% of control values, respectively. Peripheral blood counts rose accordingly. Cells in S and G2/M phases increased over 10 hr and then declined in number. It thus appeared that some synchronization of cell cycling occurred, which might place cells in a more radioresistant phase of the cell cycle. The glutathione (GSH) content and synthesis in BM cells were measured by isocratic paired-ion high performance liquid chromatography and 35S-labelled cysteine incorporation into the GSH tripeptide. An increase in cellular GSH content and synthesis was demonstrated following IL-1 which lasted 24 hr, suggesting a possible mechanism for the radioprotection by IL-1. To determine the potential for achieving a favorable therapeutic ratio, KHT tumor-bearing mice were injected with 2.0 micrograms IL-1. No change in tumor diameters or weights or tumor cell clonogenicity between IL-1 treated or untreated animals was observed. These experiments strongly support a role for IL-1 in stimulating bone marrow to overcome the myelosuppressive effects of irradiation.