The fact that Fc RI is not only expressed on mast cells and basophils but also Langerhans cells (LC) led us to investigate the expression of Fc RI and IgE bound Fc RI on LC in atopic dermatitis by immunohistochemical staining using new monoclonal anti Fc RI antibodies, AER 24 and AER 37. AER 24 can block IgE binding to Fc RI and detect only IgE free Fc RI, while AER 37 can not and detect all Fc RI. LC were determined by CD1a expression. CD1a, AER 24 and AER37 positive cells were counted per unit of epidermal area (number/mm 2 ) in 12 atopic individuals and 5 normal controls. Our results showed that the number of Fc RI expressed LC in atopic individuals (63.6±40.8) was significantly larger than that in normal controls (7.0±3.5), and that about half of those Fc RI on LC of atopic individuals had bound IgE. The number of CD1a positive epidermal LC was significantly smaller in atopic individuals (103.3±18.6) than that in normal controls (169.0±24.2) suggesting that LC may move to dermis from epidermis for antigen presentation. From these results, it is concluded that IgE dependent activation of LC may play an important pathogenic role in atopic dermatitis.