Most studies of erythropoiesis in vitro have employed cloning methods in semisolid medium. We have recently described a two-step liquid culture procedure that supports the proliferation and differentiation of human erythroid progenitors. In the present study, we have modified the procedure to allow large-scale cultures of erythroid cells derived from normal donors. The culture is divided into two phases. In the first phase, which is erythropoietin (Epo) independent, the early erythroid progenitors multiply and differentiate. In the second, Epo-dependent phase, they mature into orthochromatic normoblasts and enucleated erythrocytes. Using this procedure, erythroid cell yield reached 7.5 × 106/ml and a total of 7 × 108 cells could be harvested per blood unit. A comparison of the growth of erythroid cells in liquid culture to their colony growth in semisolid culture indicated that cell growth was superior: 1) in liquid culture in terms of cell yield per originally cultured mononuclear cell, 2) per ml culture and per culture surface area and in the purity of the resultant erythroid cell population. In addition, it permits easier manipulation of the culture condition and components and sampling of > 1 × 107 cells at each maturation stage subsequent to the proerythroblast stage. This liquid culture procedure might provide an important experimental tool for studying erythroid cell development.