AbstractCells 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 self‐renewal capacity may be an intrinsic property of the primary mixed‐erythroid colonies has been shown by the 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 mixed‐erythroid colony) than bone marrow‐derived mixed‐erythroid colonies (no secondary mixed‐erythroid colonies obtained) suggesting that external factors may predetermine the 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