Iron chelators have been implicated to modulate certain inflammatory mediators and regulate inflammatory processes. Here we report that iron chelator deferoxamine (DFO) induces differentiation of monocytic THP-1 cells into functional macrophages. DFO rapidly phosphorylated both extracellular signal-regulated kinase (ERK) and p38 kinase. Blockade of ERK signaling by the MEK1/2 inhibitor PD098059 abolished DFO-induced class A scavenger receptor (SR-A) expression and phagocytic activity, indicating that ERK cascades mediate the induction of THP-1 differentiation. In contrast, in cells treated with the p38 inhibitor SB203580 or transfected with the dominant-negative variant of p38 kinase, DFO-mediated ERK activation became more prominent, and the induction of SR-A expression and phagocytic activity were significantly increased. Interestingly, differentiation by DFO was associated with decrease in cellular glutathione (GSH) level. Both MAPK inhibitors did not influence the GSH level; however, treatment with ferric citrate (Fe3+) or N-acetyl-cysteine, a major precursor of GSH, markedly recovered GSH level to a normal extent, along with the significant decrease of differentiation. Collectively, these results indicate that oxidative stress by DFO and the resulting activation of ERK cascade play dominant roles in the process of THP-1 differentiation, while p38 acts as a negative signal transmitter.