Erythroid progenitor cells differentiate in vitro. Erythroid progenitor cells were induced to differentiate in the presence of Epo and insulin (see Materials and Methods). After 1, 2, 4, and 5 days of differentiation, cells were subjected to cytocentrifugation and staining with neutral benzidine and histological dyes and photographed. Day 0, undifferentiated cells cultured under standard growth conditions. Please note the enucleated cells obtained at day 4 to 5 of differentiation.