T o examine the influence of insulin and insulinlike growth factor (IGF) on erythropoiesis, we tested their effects in human bone marrow cultures prepared with biochemically defined medium or a platelet-poor plasma-derived serum (PDS) that was depleted of hormones by adsorption to activated charcoal. Erythroid colony formation was enhanced two- to threefold by 10 ng/ml of electrophoretically pure IGF-II and 100 ng/ml of highly purified insulin (P less than 0.05). Dose-response curves for IGF-II were parallel to and shifted by one to two orders of magnitude to the left relative to those for insulin. When added together to culture, IGF-II and insulin expressed additive activities. In contrast, their activities were synergistic with those of erythropoietin and burst-promoting activity. The erythropoietic actions of IGF-II and insulin were similar in PDS and whole blood serum (WBS) containing cultures. Furthermore, when added to cultures with electrophoretically pure platelet-derived growth factor, their respective activities were synergistic. We conclude that insulin and IGF-II potentiate human marrow erythropoiesis in vitro. Their activities appear to be mediated by a similar receptor or postreceptor system.