Human and murine studies showed that granulocyte macrophage colony-stimulating factor (GM-CSF) exerts beneficial effects in intestinal inflammation. To explore whether GM-CSF mediates its effects via monocytes, we analyzed effects of GM-CSF on monocytes in vitro and assessed the immunomodulatory potential of GM-CSF-activated monocytes (GMaM). We used microarray technology and functional assays to characterize GMaM in vitro and used a mouse model of colitis to study GMaM functions in vivo. Peripheral blood monocytes where cultured 16 h with media containing AB serum (control monocytes) or media containing 10 ng/mL GM-CSF and AB serum (GM-CSF activated monocytes). In three independent experiments, total RNA from GMaM and control monocytes was isolated and processed for microarray hybridization.