AbstractMedia conditioned by fibroblast‐like cells derived from organs active in fetal lymphohematopoiesis were studied for their effects on adult gran‐ulocyte/macrophage colony‐forming units (CFU‐GM). Fibroblasts from fetal liver produced a factor stimulatory for CFU‐GM, whereas fibroblasts from fetal marrow produced a factor inhibitory for CFU‐GM which was not completely relieved by adding indomethacin to the assay. Our studies indicated that neither fetal marrow nor fetal liver produced factors affecting lymphocyte colony‐forming units (CFU‐L). Cell‐cell interactions between fibroblast‐like cells derived from fetal liver or marrow and normal adult CFU‐GM were also studied. We observed that fibroblasts derived from both fetal and adult marrow inhibited colony formation, whereas inhibition in the presence of fetal liver fibroblasts was minimal. Loss of inhibitory activity by a liver fibroblast cell line over repeated passages was seen. Differential analysis of colonies formed above an adherent layer of fetal marrow fibroblasts suggested that these fibroblasts suppress myeloid/macrophage differentiation to a far greater degree than did adult marrow fibroblasts. A role in the regulation of fetal lymphohematopoiesis may be played b