To determine whether inhalant allergens could induce eczematous lesions we studied 17 patients with atopic eczema (with or without allergic rhinitis), 13 patients with allergic rhinitis without atopic eczema and 10 healthy control subjects. The allergens, birch pollen (Betula verrucosa) and house dust mite (Dermatophagoides pteronyssinus), were applied in aluminium chambers for 48 h on clinically normal skin. In 17 patients with atopic eczema, six epicutaneous test reactions of the delayed type to birch pollen and three to house dust mite were seen at 48 or 72 h. In 13 patients with allergic rhinitis without eczema there was one delayed reaction to birch pollen and none to house dust mite. No delayed type test reactions to either allergen were seen in the controls. Biopsies of the positive test sites revealed an eczematous reaction with epidermal spongiosis and microvesiculation. Immunostaining of cryostat sections showed dermal cell infiltrates consisting of mainly T lymphocytes (ratio of T4:T8, 2-6:I) and to a lesser degree Langerhans and indeterminate T6+ cells. 50-90% of the cells were Ia+. The numbers of basophils and mast cells did not exceed 10-15%.