The role of ferritin expression on the labile iron pool of cells and its implications for the control of cell proliferation were assessed. Antisense oligodeoxynucleotides were used as tools for modulating the expression of heavy and light ferritin subunits of K562 cells. mRNA and protein levels of each subunit were markedly reduced by 2-day treatment with antisense probes against the respective subunit. Although the combined action of antisense probes against both subunits reduced their protein expression, antisense repression of one subunit led to an increased protein expression of the other. Antisense treatment led to a rise in the steady-state labile iron pool, a rise in the production of reactive oxygen species after pro-oxidative challenges and in protein oxidation, and the down-regulation of transferrin receptors. When compared to the repression of individual subunits, corepression of each subunit evoked a more than additive increase in the labile iron pool and the extent of protein oxidation. These treatments had no detectable effects on the long-term growth of cells. However, repression of ferritin synthesis facilitated the renewal of growth and the proliferation of cells pre-arrested at the G1/S phase. Renewed cell growth was significantly less dependent on external iron supply when ferritin synthesis was repressed and its degradation inhibited by lysosomal antiproteases. This study provides experimental evidence that links the effect of ferritin repression on growth stimulation to the expansion of the labile iron pool.