The effect of long-term regular and moderate exercise on antibody production has been studied in mice. Healthy mice were trained to run for 10 min twice a day, and antibody levels following immunization were compared with those of healthy sedentary control mice. The antibody levels of the running mice were significantly higher than those of the sedentary animals at all times tested, from 1 to 13 wk post immunization. Overall, the antibody titres of the runners were 2.76 times that of the controls. The significance of these findings is discussed.