Arginine, initially classified as a non-essential amino acid, participates to multiple biological processes including release of several hormones, collagen synthesis during wound healing, antitumor and antibacterial activities and non-specific immunity. Nitric oxide synthase and arginase competes for L-arginine as a substrate and this event appears to play a key role in the regulation of the inflammatory process. In this framework recent studies have identified complex patterns of interactions among these enzymes. This review will emphasizes some effects of L-arginine on immune cell functions, including triggering of L-arginine-nitric oxide and arginase pathways, its biological properties and therapeutical applications.