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Lymphocyte activation culminates in blastogenesis, cell cycle progression, DNA replication and mitosis. These complex cellular changes are programmed almost simultaneously by multiple ligands and receptors that trigger specific signal transduction pathways and transcription factors. Until now, the discovery of the genes regulated by each ligand/receptor pair has been hampered by the technologies available. To identify interleukin-2 (IL-2)-responsive genes, human peripheral blood mononuclear cells (PBMC) were pre-activated with anti-CD3, rested, and restimulated with IL-2 for 4 hr. Gene expression was analyzed using Affymetrix U95Av2 oligonucleotide arrays. To determine the most stringent parameters to score a gene as a bona fide IL-2 target, the expression of 19 known IL-2-regulated genes was examined first. All were induced at least 2-fold, with a difference in fluorescent intensity of ≥ 100 at p < 0.05. An additional 53 unique genes met these criteria. To determine which of these were immediate/early IL-2 targets in T cells, purified T cells were stimulated with IL-2 for 4 hr in the presence of cycloheximide to prevent secondary gene expression. Of the 72 genes identified in PBMCs, 20 were detected as immediate/early IL-2-regulated genes in purified T cells. In addition, 27 unique genes were IL-2-regulated in T cells but not in PBMCs. For a successful reductionist approach to the analysis of gene expression in lymphocyte activation, it is necessary to examine purified cell populations and immediate/early gene expression regulated by each ligand/receptor pair involved. This approach should allow the discovery of genes regulated by all of the ligand/receptor pairs involved in lymphocyte activation.