Twenty-four T cell clones (TCC) specific for purified Dermatophagoides pteronyssinus group I allergen (Der p I) were established from the peripheral blood of a patient with severe atopic disease and assessed for cytokine production in response to stimulation with both phorbol 12-myristate 13-acetate and anti-CD3 monoclonal antibody. All Der p I-specific TCC produced high amounts of interleukin (IL)-4 in association with IL-3, IL-5 and granulocyte monocyte-colony stimulating factor (GM-CSF), whereas they produced variable amounts of IL-2 and virtually no interferon-gamma. These data support the hypothesis that atopy is associated with preferential activation of type 2 T helper cells and suggest a deregulation in the function of the IL-3, IL-4, IL-5 and GM-CSF gene cluster in subjects with severe atopic disorders.