The quantitative measurement of the IgE and IgG subclass levels in the sera of 29 patients with atopic dermatitis has revealed significantly elevated levels of IgE and IgG4 in a relatively high proportion of patients with this condition compared to non-atopic patients with warts and to a group of normal volunteers studied previously. The possible significance of these observations is discussed in relation to current evidence suggesting the involvement of IgG4 in certain immediate-type hypersensitivity disorders.