Although the association of erythroblasts with macrophages has been well documented in the human bone marrow, the function and identification of the intimate contacts occurring between the membranes of these two cell types in the physiology of erythropoiesis is not known. Using in vitro cultures of human peripheral blood derived erythroid progenitors, we have shown the presence of erythroblastic islands consisting of a central macrophage surrounded by a ring of erythroblasts that undergo terminal maturation leading to enucleation. However, when cultures were carried in the absence of intact macrophages, erythroid cells matured to the late erythroblast stage but failed to enucleate. Furthermore, the number of erythroid cells was markedly reduced in macrophage-depleted cultures, suggesting that the erythroblast-macrophage contact promotes proliferation and terminal maturation of erythroid cells leading to their enucleation. To examine the molecule(s) involved in the interaction between erythroblasts and macrophages, we have used a cell attachment assay involving incubation of solubilized surface-labeled erythroblasts with macrophage membrane proteins separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to a nitrocellulose membrane. Erythroblast surface proteins specifically attached to a 30-kD protein from macrophage membranes, whereas no adhesion was seen to the protein standards. An apparently similar protein of 30 kD was also detected on erythroblasts and was shown to mediate erythroblast-erythroblast contact in addition to the erythroblast-macrophage contact. The extraction of plasma membranes with Triton X-100 showed that the 30-kD protein is linked to the membrane skeleton via an integral membrane protein both in erythroblasts and macrophages. Furthermore, our results show that the cell:cell interactions mediated by the 30-kD protein are calcium-independent and could be specifically inhibited by heparin. We conclude that the association of erythroblasts with macrophages promotes erythroid proliferation and maturation leading to erythroblast enucleation and that a 30-kD heparin-binding protein present on the surface of macrophages and erythroblasts is involved in this contact. This protein is capable of binding homotypic and heterotypic cells.