Taurine is a semiessential amino acid that is not incorporated into proteins. In mammalian tissues, taurine is ubiquitous and is the most abundant free amino acid in the heart, retina, skeletal muscle, and leukocytes. Taurine reaches up to 50 mM concentration in leukocytes. Taurine has been shown to be tissue-protective in many models of oxidant-induced injury. One possibility is that taurine reacts with HOCl, produced by the myeloperoxidase (MPO) pathway, to produce the more stable but less toxic taurine chloramine (Tau-Cl). However, data from several laboratories demonstrate that Tau-Cl is a powerful regulator of the immune system. Specifically, Tau-Cl has been shown to downregulate the production of proinflammatory mediators in both rodent and human leukocytes. Recent molecular studies on the function of taurine provide evidence that taurine is a constituent of biological macromolecules. Specifically, two novel taurine-containing modified uridines have been found in both human and bovine mitrochondria. In studies on mechanism of action, Tau-Cl inhibits the activation of NFkappaB, a potent signal transducer for inflammatory cytokines, by oxidation of IkappaB alpha at methionine45. Taurine transporter knockout mice show reduced taurine, reduced fertility, and loss of vision resulting from severe retinal degeneration, which was found to be due to apoptosis. Apoptosis induced by amino chloramines is a current and important finding because oxidants derived from leukocytes play a key role in killing pathogens. The fundamental importance of taurine in adaptive and acquired immunity will be revealed using genetic manipulation.