Monocytes/macrophages serve as sentinels involved in chronic inflammation and the eradication of various pathogens. To define molecularly the differentiation of blood monocytes into macrophages, we conducted serial analysis of gene expression (SAGE) in human blood monocytes/macrophages induced by granulocyte-macrophage colony-stimulating factor (GM-CSF) or M-CSF. SAGE analysis of 57,560, 57,463, and 55,856 tags from monocytes, GM-CSF–, and M-CSF–induced macrophages, respectively, allowed identification of 35,037 different transcripts. Interestingly, the genes with the highest expression during differentiation from monocytes into macrophages were genes involved in lipid metabolism. Both CSF-induced macrophages expressed similar sets of genes except for several genes such as monocyte-derived chemokine (MDC), legumain, prostaglandin D synthetase, and lysosomal sialoglycoprotein. The identification of specific gene expression in human monocytes, GM-CSF–, or M-CSF–induced macrophages provides novel methods to define macrophage subsets and the maturation and activation stage of cells of macrophage lineage and, possibly, to diagnose diseases in which macrophages play a major role. This study represents the first extensive serial analysis of gene expression for any type of human hematopoietic cells.