The immunomodulatory activity of ginsenoside Rg₁ from Panax ginseng was studied in mice using sheep red cells as the antigen. It was found that ginsenoside Rg₁ at a dose of 10 mg/kg administered for three consecutive days before immunization increased the number of spleen plaque-forming cells, the titers of sera hemagglutinins as well as the number of antigen-reactive T-cells. Ginsenoside Rg₁ also increased the number of T-helper cells with respect to the whole T-cell number and the splenocyte natural killer activity. Ginsenoside Rg₁ induced an augmentation of the production of IL-1 by macrophages and exerted a direct mitogenic effect on microcultured thymus cells. Ginsenoside Rg₁ also partly restored the impaired immune reactivity by cyclophosphamide treatment.