Zinc is necessary for the normal functioning of all living systems. In microbes and mammals vital biological roles of zinc include the presence of this metal in metalloenzymes and membrane stabilization. In addition, zinc may bind nonspecifically to sulfhydryl, histidine, or other moieties on membranes, enzymes, or other proteins and alter their activity. Serum levels of zinc decrease sharply in many infections. Levels slightly below normal seem to be associated with optimal phagocytic function, and low concentrations of zinc may decrease microbial virulence. Brief decreases in serum levels appear to have no detrimental effect on host immunity and may act as a protective measure by decreasing the ability of indigenous or infecting microbes to thrive. However, prolonged zinc deficiency in mammals is associated with depressed T-cell function but near normal B-cell function. More work is needed for better understanding of zinc-related biochemistry and for determining the relationship between zinc status and susceptibility to infection in mammals.