Macrophages can metabolize arginine to nitric oxide in quantities that inhibit pathogens or nearby host cells. They can instead metabolize arginine to ornithine (a precursor of polyamines and collagen) in quantities that stimulate pathogens or nearby host cells. Macrophages are essentially the only circulating cells that can make these life or death decisions with arginine. Macrophages expressing these destructive or constructive phenotypes have been termed M-1 or M-2 because they also stimulate T H 1 or T H 2 responses, respectively. Factors that influence whether a macrophage expresses the M-1 or M-2 phenotype and the real or potential impact on immune responses and other host processes are discussed.