Differentiation of p19-induced 32D cells. Parental 32D cells (A) and the 2C subclone (B) maintained in medium containing IL-3 but lacking zinc have the appearance of myeloid blast cells. When shifted to medium containing 75 μmol/L zinc (C, D) and examined 4 days later, the cells acquired a macrophage-like morphology (C) and a fraction of them exhibited Fc-receptor-dependent immunophagocytosis (D). When transferred to medium containing G-CSF instead of IL-3, p19-inducible clones exhibited nuclear changes typical of neutrophils after only 3 days of culture (E). Under identical conditions in medium containing both G-CSF and zinc, some macrophage-like cells also emerged (arrowheads) (F ). If clone 2C cells were grown in G-CSF plus zinc for 4 days and shifted back to medium containing IL-3 plus zinc, macrophage-like cells but not neutrophils persisted, and by 7 days after medium change, the cultures resembled those illustrated in (C). No macrophage-like cells were ever observed in cultures of parental 32D cells grown in G-CSF plus zinc (negative data not shown).