Aging leads to a diminution of resting immune function, increasing the risk of infection, tumor development and auto-immune diseases. The production of interleukin-2 is decreased, sometimes with a decrease of total T cell count, and often with changes in T cell subsets and proliferative responses to mitogens. However, natural killer cell activity remains unchanged. In theory, moderate exercise should help to reverse the adverse effects of aging upon the immune system. However, there have been relatively few studies comparing the immune responses of young and older individuals to acute exercise and to training. A single bout of moderate exercise seems to be well tolerated by the elderly. The NK cell response is much as in younger individuals, but perhaps because of a low initial proliferative capacity, older subjects show less stimulation of lymphocyte proliferation by moderate activity and less suppression with exhausting exercise. Perhaps because resting immune function is less than in the young, moderate training programs seem to stimulate immune function to a greater extent than in young subjects. The proliferative response of the T cells is enhanced in elderly rodents whereas in young animals it is suppressed. Moreover, the resting NK cell activity of elderly human subjects seems to be increased by training. Nevertheless, the therapeutic use of exercise must be cautious in the elderly, since aging also enhances susceptibility to over-training.