Experiments were performed on mice to investigate the influences of ampicillin and ethanol on the immune response. Ampicillin was injected intraperitoneally and ethanol was administered in the drinking water. Mice were sensitized and challenged with sheep red blood cells. Immune responses were evaluated by humoral immunity, cellular immunity, peripheral circulating white blood cell and phagocyte activity. 1. The combined administration of ampicillin and ethanol as compared to ampicillin had not influence on the weight of spleen, but increased the weight of thymus. 2. Humoral immune response was slightly reduced by ampicillin. Especially, the combined administration of ampicillin and ethanol significantly reduced hemclysin production. 3. Cellural immune response was reduced by ampicillin. The combine administration of ampicillin and ethanol significantly reduced cellural immune response. 4. Peripheral circulating white blood cell was reduced by the combined administration of ampicillin and ethanol as compared to ampicillin. 5. The combined administration of ampicillin and ethanol as compared to ampicillin had not influence on the phagocyte activity.