The ginsenoside Rg1 extracted from the root of Panax ginseng can promote mitosis in cultured human lymphocytes activated by PHA or Con A. Its most effective concentrations are around 0.0003-0.0005 mg per ml of medium. Experiments show that it does not arrest the cells at any particular mitotic stage. It can also enhance the DNA synthesis in the activated lymphocytes. As a result of the increased number of the mitotic cells and enhanced DNA synthesis, the cell density is significantly increased in the Rg1-treated culture as compared with the control. However, in the absence of a mitogenic lectin Rg1 cannot restart the quinescent human lymphocytes to divide in vitro; therefore it is not mitogenic to resting cells. The possible action of Rg1 on activated human lymphocytes as well as its pharmacological significance are discussed.