It has been well documented that the immune function declines with age in the human and animals. The possible causes for the decline are the inability of lymphocytes to proliferate in response to mitogenic stimulation and the decrease of IL-2 production. In the present studies, Rg1 was shown to selectively enhance the proliferation of lymphocytes and the production of IL-2 in aged rats. Using Northern blot and Western blot analyses, Rg1 was found to promote IL-2 gene expression which showed increase of IL-2 mRNA and IL-2 protein contents. Interestingly, under the same conditions, studies were carried out on the effect of Rg1 on the immune function of young adult rats, but no marked influence was observed. According to these results, it is reasonable to consider Rg1 an immunoregulator rather than a purely "immunopotentiating agent".