Bone marrow, with its intricate, three-dimensional tissue structure facilitating cell-cell interactions, provides a microenvironment supporting the production of hundreds of billions of multilineal blood cells everyday. We have developed a three-dimensional bone marrow culture system in which marrow cells are cultured in a reactor packed with porous microspheres. The culture supports a three-dimensional growth configuration and multilineal hemopoiesis mimicking the bone marrow in vivo. We studied ex vivo human erythropoiesis using the three-dimensional culture system. The system sustained extensive erythropoiesis at low erythropoietin concentrations (0.2 U/mL), plus stem cell factor, interleukin-3, granulocyte-macrophage colony-stimulating factor, and insulin-like growth factor-I. Erythroid cell production lasted for more than 5 weeks, and the percentage of erythroid cells in the nonadherent cell population was approximately 60%. Flow cytometric analysis using cell surface markers specific for erythroid cells (CD71 and glycophorin-A) indicated that the culture produced early, intermediate, and late erythroid cells. As the culture progressed, the erythroid cell population shifted gradually toward mature cell types. When compared to the three-dimensional culture, the traditional flask cultures failed to support extensive erythropoiesis under the same conditions. This indicates that the three-dimensional bone marrow culture system provides a microenvironment conducive to erythropoiesis under more physiological conditions and is a better bone marrow model.