The normal regulation of primitive hemopoietic stem cells (PHSCS) throughout development involves locally produced factors and humoral factors. While considerable information is available on the effects of candidate hemopoietic growth factors (HGFs) on postnatal hemopoietic tissues, little is known about the regulatory events of developing hemopoietic stem cells within fetal microenvironments. Fetal hemopoiesis represents expanding populations and may be under different regulatory control mechanisms. The microinjection of purified interleukin-3 (IL-3), a candidate HGF, into 13-day-old mouse fetuses via the yolk sac, allowed us to evaluate its effects on morphogenetic events and, more specifically, on fetal liver populations using quantitative in vitro clonal assays for hemopoietic precursors. In view of the sensitivity of fetal development during the early organogenetic period, considerable care was taken to identify the stress effects of the surgical laparotomy and the microinjection procedure. Control studies, required to distinguish stress effects of surgical laparotomy and microinjection, clearly revealed that the fetal liver is a sensitive organ responding with limited tissue disorganization, reduced cellularity and erythropoietic activity, as monitored 24 h after experimental intervention. The microinjection of 15 units of IL-3 promoted a significant expansion of depleted liver hemopoietic-cell populations and had stimulatory effects on the distribution of connective tissue mast cells and absolute cell numbers, including hemopoietic precursors (erythroid, granulocyte, macrophage, megakaryocyte), compared to controls. These studies suggest that (1) fetal liver hemopoiesis is selectively sensitive to maternal stress but has an effective regenerative capacity to maintain essential hemopoiesis in utero, and (2) fetal hemopoietic cells acquire an ability to respond to IL-3 early in fetal development.