Macrophages (Mϕ) represent dynamic cell populations that develop according to the nature of environmental signals. It has been demonstrated that human Mϕ can be polarized in vitro into pro-inflammatory (Mϕ1) and anti-inflammatory cells (Mϕ2) by the lineage-determining factors GM-CSF and M-CSF, respectively. Here we show that polarized Mϕ1 and Mϕ2 are not an end stage of differentiation but are able to reversibly undergo functional re-differentiation into anti-inflammatory and pro-inflammatory Mϕ. GM-CSF-driven Mϕ1 exposed to M-CSF for an additional 6 days obtained a Mϕ2-like phenotype, inhibited the production of pro-inflammatory cytokine IL-6 and TNF-α, and exhibited a reduced T cell stimulatory capacity. Vice versa, Mϕ2 exposed to GM-CSF exhibited a Mϕ1-like phenotype with significant lower production of anti-inflammatory cytokine IL-10 and a higher T cell stimulatory activity, and a decreased capacity for phagocytosis of early apoptotic cells. Our data suggest that polarized macrophages are flexible in modulating their immune functions upon environmental changes, i.e., steady-state versus inflammatory conditions. These observations are important for our understanding of the regulatory role of macrophages in tissue homeostasis and disease pathogenesis.