Injection of a single dose of recombinant human interleukin-1 alpha (r-hu-IL-1 alpha) into mice 24 hr after 5-fluorouracil (FU) treatment resulted in an increased rate of recovery of three types of colony-forming cells (CFCs) in the bone marrow. Myeloid progenitors with high proliferative potential (responsive to CSF-1 + IL-3 + IL-1 alpha), low proliferative potential (responsive to CSF-1), megakaryocyte progenitors, and total nucleated cells per femur increased up to 5-fold, 7-fold, 3-fold, and 3-fold, respectively, in a dose related fashion compared with the control FU treated marrows. The kinetics of FU kill and recovery of these CFCs are shown.