The induction of IgE antibodies to aeroallergens depends upon antigen-specific CD4+ helper T cells of an'interleukin-4 (IL-4)-dominant'phenotype. Nuts also drive IgE-mediated hypersensitivity and are the most dangerous of the orally encountered allergens. We have studied the polyclonal T-cell responses of atopic and non-atopic individuals to extracts of peanut, brazilnut and hazelnut. Strong proliferative responses were observed in all patients but specific IgE was only present in the nut-allergic patients suggesting a similar pathogenic mechanism to aeroallergen-mediated hypersensitivity. To investigate this hypothesis a panel of peanut-reactive T-cell clones was raised from a peanut-and brazilnut-allergic individual without hazelnut allergy. The antigen specificity, major histocompatibility complex (MHC) class II restriction and cytokine profiles of the T-cell clones were determined. With the exception of one T-cell clone, which proliferated in response to both peanut and hazelnut extract, the peanut T-cell clones were not cross-reactive with hazelnut or brazilnut. The T-cell clones recognized antigen in association with HLA-DR and HLA-DP but not HLA-DQ class II molecules. The peanut-specific clones produced high levels of IL-4 and low levels of interferon-gamma (IFN-gamma), exhibiting the'TH2-like'profile which dominates the aeroallergen response. In contrast, the T-cell clone that was cross-reactive on both peanut and hazelnut allergen had a Th0-like phenotype, consistent with the lack of specific serum IgE to hazelnut. These results support the importance of functionally distinct T-cell populations that recognize oral allergens. The relative production of IL-4 and IFN-gamma of the cloned T cells in the peanut-allergic patients plays a role in determining whether or not IgE antibody responses are induced with the associated potential to develop anaphylactic reactions.