Granulocyte-macrophage colony-stimulating factor (GM-CSF), a 23-kDa polypeptide, was originally identified as a hematopoietic growth factor, but has recently been found to be a multifunctional cytokine with many proinflammatory activities (1,2). GM-CSF can be produced by, and act upon, a broad range of cell types, including both immature and mature granulocyte and monocyte lineage cells, dendritic cells, and tissue structural cells. Abundant in vitro observations have suggested that GM-CSF is able to induce both differentiation and activation of these cells (1).