Plasma prolactin has been shown to increase during stress; the immune system is responsive to prolactin and affected by stress. Therefore, this study was undertaken to investigate the effects of acute graded, maximal treadmill exercise on prolactin-receptor expression by lymphocytes. Eight healthy men underwent one exercise and one nonexercise session. Blood was sampled immediately before and after the exercise. On the day of the nonexercise session, two resting blood samples were obtained at the same times as the exercise session samples to act as baseline data. Plasma prolactin concentrations were significantly elevated in response to exercise and correlated positively with total prolactin-receptor expression per B lymphocyte. An increase in total prolactin-receptor expression per B lymphocyte in response to exercise also was observed. In addition, exercise significantly increased the total number of circulating B lymphocytes expressing prolactin receptor as well as the total number of circulating B lymphocytes. These data support the idea that exercise may enhance the interaction between immune target cells and prolactin, astress hormone capable of enhancing immune function.