Arginine is a semiessential dibasic amino acid for humans and animals. It is synthesized from other amino acids via the urea cycle, but exogenous arginine is required during times of anabolism (immaturity and injury) and/or stress (sepsis, trauma, and nitrogen overload) to ensure optimal growth and maintain a positive nitrogen balance. Nitric oxide (NO) or a similar nitroso compound derived from arginine via a deaminase reaction has been characterized in a large and growing number of mammalian tissues including endothelial cells, macrophages, neutrophils, T lymphocytes, mast cells, Kuppfer cells, adrenal tissue, and neurons. In general, arginine is required for mammalian cell culture growth. Arginine has many and varied biological properties including immunostimulation. Multiple effects of arginine on immune function involve endocrine, paracrine, and autocrine mechanisms. More research is needed to differentiate between those effects of arginine that require metabolism to NO and those that require conversion to ornithine.