One of the physiological effects of human growth hormone (hGH) is regulation of certain immune functions. In addition, lymphocytes can produce hGH which may serve as an autocrine growth factor. In this study, we have evaluated the role of hGH as a mitogen and autocrine growth factor. We have shown that hGH like other classic mitogens can stimulate lymphocyte proliferation, expression of activation antigens (receptors for interleukin-2 and transferrin) and secretion of interleukin-2 by lymphocytes (P < 0.05). Activated lymphocytes themselves have the capacity to secrete hGH, with secretion peaking on the 7(th) day, while anti-hGH polyclonal serum inhibited proliferation of lymphocytes (P < 0.05). These data provide evidence for the important role of hGH in lymphocyte activation and proliferation.