Cells obtained from in vitro colonies expressing multiple hemopoietic lineages (mixed-erythroid colonies) have been shown to produce secondary mixed-erythroid colonies. When individual mixed-erythroid colonies are studied, considerable variation has been observed in the capacity to produce secondary mixed-erythroid colonies. That this heterogeneity in selfrenewal capacity may be an intrinsic property of the primary mixed-erythroid colonies has been shown by t h e ability to fractionate fetal liver mixed-erythroid colony-forming cells with high and low levels of secondary colony formation. In addition mixed-erythroid colony-forming cells obtained from spleens consistently produce higher numbers of secondary mixed-erythroid colonies (up to 250 secondary mixed-erythroid colonies per primary mixederythroid colony) than bone marrow-derived mixed-erythroid colonies (no secondary mixed-erythroid colonies obtained) suggesting that external factors may predetermine t h e ability of primary mixed-erythroid colony-forming cells to self-renew in vitro. The ability of supernatants obtained from long-term marrow cultures to enhance secondary colony formation by bone marrow-derived mixed-erythroid colony-forming cells raises the possibility of defining the nature of the proposed external factors that determine stem cell self-renewal.