Exercise responses are discussed with particular reference to the functional system involving interleukin-1, interleukin 2 and cytotoxicity. Prolonged endurance exercise causes an increase in plasma levels of interleukin-1, possibly as a response to muscle injuries, but plasma interleukin-2 levels generally fall. The latter change probably reflects stronger binding, consequent upon increased expression of p70-75 receptors for IL-2, and changes in the distribution or activity of target cells; IL-1 secretion may enhance the responsiveness of peripheral blood mononuclear cells, but prostaglandin secretion decreases their IL-2 production. Moderate exercise increases the cytolytic action of NK cells, but there is a prolonged fall of cytolytic activity after exhausting or psychologically stressful exercise; again these responses probably reflect altered IL-2 levels or receptor expression. Appropriately graded training reduces the adverse immune reactions associated with challenging exercise. Cross-sectional comparison and training experiments both show an increased expression of p70-75 IL-2 receptors on the NK cells of active individuals. Moreover, moderate training reduces the exercise-induced suppression of IL-2 production. However, training that is pursued to the level of staleness, nutrient deficiency or muscle damage has a negative impact upon both the production of interleukins and the leukocyte response. Coaches must thus gauge training programs to optimize not only physiological function but also immune responses.