Activated macrophages require L -arginine uptake to sustain NO synthesis. Several transport systems could mediate this L -arginine influx. Using competition analysis and gene-expression studies, amino acid transport system y + was identified as the major carrier responsible for this activity. To identify which of the four known y + transport-system genes is involved in macrophage-induced L -arginine uptake, we used a hybrid-depletion study in Xenopus oocytes. Cationic amino acid transporter (CAT) 2 antisense oligodeoxyribonucleotides abolished the activated-macrophage-mRNA-induced L -arginine transport. Together with expression studies documenting that CAT2 mRNA and protein levels are elevated with increased L -arginine uptake, our data demonstrate that CAT2 mediates the L -arginine transport that is required for the raised NO production in activated J774 macrophages.