L-arginine metabolism in myeloid cells plays a central role in the processes of macrophage activation and in the regulation of immune responses. In this study, we investigated arginine transport activity and the expression of the related transporter genes during the differentiation of monocytes to macrophages. We show here that the induction of THP-1 monocyte differentiation by PMA markedly increases the expression of SLC7A7 mRNA and of y(+)LAT1 protein and consequently, the activity of system y(+)L-mediated arginine transport. Conversely, the activity of system y(+) decreases during macrophage differentiation as a result of a reduction in CAT1 protein expression. The PMA-induced, macrophage-differentiated phenotype and the increased activity of system y(+)L through the induction of SLC7A7 gene are mediated by the specific activation of PKCβ. SLC7A7 gene silencing causes a significant reduction of system y(+)L activity and a subsequent, marked increase of arginine and lysine cell content, thus suggesting that in macrophagic cells, system y(+)L activity is mainly directed outwardly. Differentiating agents other than PMA, i.e., VD3 and ATRA, are equally effective in the stimulation of system y(+)L transport activity through the increased expression of SLC7A7 mRNA and y(+)LAT1 protein. Moreover, we found that also during differentiation of human monocytes from peripheral blood SLC7A7 mRNA and system y(+)L activity are increased. These findings point to SLC7A7 gene as a marker of macrophage differentiation.