We used the saponin Rg1 extracted from Panax ginseng to study its effects on lymphocytes of 10 young and 19 elderly persons. The proliferative response of lymphocytes cocultured for 72 h with PHA and saponin was measured by using MTT method and the 3H-TdR incorporation procedure. PHA and Rg1 had stimulative effects on the phenotype of lymphocytes (P < 0.001). Rg1 also increased the fluidity of lymphocyte membrane of the aged (P < 0.001). The CD25 and CD45RA positive cells of lymphocytes in the elderly were lower than those of the young people, 8.6% +/- 2.7% vs 10.43% +/- 3.5%; 20.95% +/- 15.5% vs. 50.86% +/- 4.2%, respectively. More CD45RO positive cells (41.5% +/- 13.9%) than CD45RA positive cell lymphocyte populations were seen in the aged. The CD45RO positive cells of the young people were 39.63% +/- 3.2%. We discussed the cause of declined immune function of lymphocytes of aged person and the mechanism of the effect of P. ginseng on lymphocytes.