We studied the effects of six cycles of repeated cyclophosphamide (CTX) therapy followed by restorative therapy with either granulocyte- macrophage colony-stimulating factor (GM-CSF) or G-CSF on the hematopoietic stem cell compartment. Stem cell function was assessed by serially transferring bone marrow cells from CTX-CSF-treated mice into lethally irradiated recipient mice. Bone marrow cells from mice that initially received either G-CSF or GM-CSF after CTX therapy more rapidly lost the ability to repopulate the recipient lymphoid organs, showed a dramatic loss of hematopoietic progenitors, a more rapid loss of CFU-S capacity, and a 40% to 50% reduction in marrow repopulating ability (MRA). Interleukin-1 (IL-1) appeared to have little effect on the CTX-treated mice when used alone. However, when administered before the CTX-CSF regimen, IL-1 prevented the stem cell depletion as determined by CFU-C, CFU-S, and MRA through the serial transplantation procedures. These results support the hypothesis that repeated treatments with myelosuppressive drugs followed by stimulation with the CSFs may induce damage to the host stem cell compartment, and further suggest that pretreatment with IL-1 before CTX therapy may prevent this stem cell damage.