Blood cells perform many important functions within the body, including homeostasis and host defense against various invading stimuli such as viral infection, cancer and autoimmune diseases. The subsets of leukocytes interact with each other through various surface molecules such as cytokine receptors, co-stimulation molecules and adhesion molecules. Over the last several years, accumulation of cDNA and genome databases has led to the accelerated identification of the molecules responsible for cell-cell interaction, cell activation and cell differentiation. In addition, technologies used in functional genomics, such as DNA microarray and serial analysis of gene expression (SAGE), have allowed us to analyze the expression of thousands of genes. The comprehensive analysis of gene expression is very useful to elucidate the function of cells, because characteristics of each cell type depend on the genes selectively expressed at various stages. The SAGE method, which is very quantitative, can cover the number of expressed genes that are unequaled by any other mammalian DNA microarray systems yet available. In order to molecularly define the subset and function of blood cells, we and other groups have performed SAGE in various types of hematipoietic cells. Here, we review the SAGE data obtained from the gene expression libraries made from various differentiation and activation stages of a broad range of blood cells, including phagocytes, T cells, B cells, platelets, reticulocytes and NK cells.