Mice of several inbred strains have been fed diets containing either large amounts of zinc (300 ppm Zn), small amounts of zinc (5 ppm Zn), or routine laboratory mouse chow. When the mice are fed on a high-zinc diet, murine strains, such as C3H/HeJ, AKR/J, and CBA/CaJ, which are normally susceptible to infection with Candida albicans and which normally release low titers of migration-inhibition factor (MIF) in vivo into the circulation, become more resistant to infection with C. albicans and release higher titers of MIF in vivo into the circulation. In addition, their capacity to elicit delayed type hypersensitivity responses may be enhanced. When the mice are maintained on a low-zinc diet, murine strains, such as C57Bl/10SNJ, which are normally resistant to infection with C. albicans and which normally release high titers of MIF in vivo into the circulation on appropriate antigenic challenge, become more susceptible to infection and release lower titers of MIF into the circulation. Under these conditions of low-zinc concentrations in the diet, their capacity to elicit delayed type hypersensitivity may be reduced. Thus, the concentration of zinc in the diet may have a pronounced effect on some in vivo parameters of cell-mediated immunity.