Flow cytometric analysis of ferritin content in cultured erythroid cells. Erythroid precursors were grown according to the two-phase liquid culture procedure. On different days of phase II (EPO-dependent phase, see Materials and Methods), cells were stained for ferritin with antihuman placental ferritin rabbit serum and antirabbit IgG labeled with FITC and for Hb with phycoerythrin-labeled antihuman HbA monoclonal Ab. Controls were stained with rabbit nonimmune serum. After 6 days in phase II, 93% of Hb-positive (erythroid) cells were also positive for ferritin (upper right quadrant) and only 7% of Hb-positive cells were ferritin-negative (upper left quadrant). After 12 days in phase II, 34% of the Hb-positive population became ferritin-negative. The plots shown are representative of three separate experiments.