The effect of red ginseng ethanol extract on the immunotoxicity of diethylstilbestrol (DES) was studied in ICR mice. ICR male mice were divided into S groups (10 mice/group), and red ginseng ethanol extract (50, 100 and 200 mg/kg body wt., respectively) and DES (1 mg/kg body wt.) were injected intraperitoneally (i.p.) to ICR mice once a day for 2 weeks. Mice were sensitized and challenged with sheep red blood cells (S-RBC). Immune response were evaluated by humoral immunity, cell-mediated immunity, non-specific immunity, and circulating leukocyte counts. The results of this study were summarized as followings: 1. The DES-treated control group as compared with normal group showed the tendency to decrease body weight rate and relative liver weight, decreased both humoral and cellular immune responses, phagocyte activity, and circulating leukocyte counts, but increased the natural killer (NK) cell activity. 2. Compared with the DES-treated control group, DES plus red ginseng ethanol extract-treated groups significantly decreased the body weight rate (P<0.01). Relative liver weight was significantly decreased in DES plus red ginseng ethanol extract (50mg/kg)-treated group (P<0.01), but significantly increased in DES plus red ginseng ethanol extract (100mg/kg)-treated group (P<0.01). Relative spleen and thymus weights were significantly enhanced in DES plus red ginseng ethanol extract (100 mg/kg)-treated group (P<0.01), but significantly decreased in DES plus red ginseng ethanol extract (200 mg/kg)-treated group (P<0.01). 3. Both humoral and cellular immune responses were significantly decreased in DES plus red ginseng ethanol extract-treated groups rather than in the DES-treated control group (P<0.01). Especially, it weakened the decrease in DES plus red ginseng ethanol extract (100 mg/kg)-treated group. 4. Phagocyte activity and circulating leukocyte counts were significantly decreased in DES plus red ginseng ethanol extract-treated groups rather than in the DES-treated control group (P<0.01). Especially, it weakened the decrease in DES plus red ginseng ethanol extract (100 mg/kg)-treated group. NK cell activity was significantly enhanced in DES plus red ginseng ethanol extract (100 mg/kg)-treated group (P<0.01), but significantly decreased in DES plus red ginseng ethanol extract (50 and 200 mg/kg)-treated groups (P<0.01).