Zinc is known to have beneficial effects on the immune response. In an attempt to modify age-associated immune dysfunction, supplemental zinc was administered to 15 subjects over 70 years of age (220 mg zinc sulfate twice daily for a month). As compared to 15 controls, matched for age and sex, there was a significant improvement in the following immune parameters in the treated group: (1) number of circulating T lymphocytes; (2) delayed cutaneous hypersensitivity reactions to purified protein derivative, Candidin and streptokinase-streptodornase; (3) immunoglobulin G (IgG) antibody response to tetanus vaccine. Zinc treatment had no influence on the number of total circulating leukocytes or lymphocytes, or on the in vitro lymphocyte response to three mitogens: phytohemagglutinin (PHA), concanavalin A (Con A) and pokeweed mitogen (PWM). The data suggest that the addition of zinc to the diet of old persons could be an effective and simple way to improve their immune function.