Normal and irradiated mice were perfused with recombinant interleukin 3 (rIL3) and the number of early hemopoietic progenitors (CFU-S) was quantified in different organs. Normal mice perfused with rIL3 for 3 or 7 days showed a dramatic increase in the number of CFU-S in the spleen, liver and blood, while the bone marrow CFU-S number was slightly decreased. The total number of CFU-S per animal was only slightly increased; the major effect of rIL3 perfusion was thus on the distribution of CFU-S in the mice. Lethally irradiated mice injected with 5 X 10(4) syngeneic bone marrow cells and perfused with rIL3 were killed after 10 days. A significant increase in both the number and size of the splenic colonies was observed. The CFU-S content of these colonies, determined in a second set of irradiated mice, was increased by more than 25-fold, indicating that in conditions of hemopoietic depletion, rIL3 vastly increased the self-renewal capacity of early progenitors.