Allergen-specific T-cell lines established from allergic patients provide the opportunity of investigating T-cell functions at the poly- or oligoclonal level. T-cell lines are useful in determining the presence or absence of antigen-specific T-cell reactivity. However, to obtain detailed knowledge of the action of T cells with clearly defined features, for example epitope specificity or phenotype, T-cell clones are necessary.The frequency of allergen-specific T cells in peripheral blood mononuclear cells (PBMC) tends to be low and so stimulation of PBMC with single allergens often results in low allergen-specific reactivity or requires high doses of the allergen. In contrast, the stimulation of PBMC with whole allergen extract results in stronger reactivity because a greater spectrum of T-cell specificities is addressed. Therefore, for the investigation of polyclonal reactivity toward single allergens it is useful to establish T-cell lines, which represent an allergen-specific enrichment of T cells from the respective individual. These T cells are poly- or oligoclonal and might possess different epitope specificities. The method described here is based on experiences with human T-cell lines and clones specific for several allergens from grass pollens and tree pollens.