We tested the hypothesis that the rate of cellular uptake of dehydroascorbate in cultures of developing granulocyte-macrophage progenitors in vitro would serve as a biochemical marker of neutrophil maturation. Suspension cultures of low-density, nonadherent, T-lymphocyte-depleted bone marrow cells from eight normal volunteers were cultured in medium containing 10% human placental conditioned medium and were harvested at intervals over 14 days. The harvested cells were tested for their ability to take up dehydroascorbate. Mean cellular uptake rate increased 12-fold by Day 10, at which time the cells had differentiated to neutrophils. Uptake increased by less than 2-fold in cells which had been induced to differentiate to mature mononuclear phagocytes with 12-O-tetradecanoylphorbol-13-acetate. Additional studies using HL-60 cells induced to differentiate with dimethyl sulfoxide or 12-O-tetradecanoylphorbol-13-acetate support the view that a major increase in dehydroascorbate uptake in cultured granulopoietic progenitors is a manifestation of a neutrophil differentiation.