The serum IgE concentration is significantly lower in aged as compared with young people. This is more pronounced in males than in females (P<0.05). In fifteen aged (>70 years) compared with fifteen young subjects (<40) the specific IgE antibody response to an injection of diphtheria toxoid was significantly reduced (P<0.01). The minimum concentration of anti-IgE antibody able to induce a weal and flare reaction when injected intradermally, was comparable in the two age groups. However, the diameter of the weal reaction was significantly reduced in the aged. It is concluded that the reduced incidence of atopic symptoms in old people might be secondary to an anomaly of both the immune system and the mast cells.