Peripheral blood mononuclear cells from atopic donors were stimulated in vitro with allergens (Rye group I or Dermatophagoides pteronyssinus). T cell lines were originated and maintained in long term culture using IL-2 and periodical restimulations with allergen. The lines were antigen specific (i.e. responded to the allergen used to raise them and not to other antigens) and required that the antigen was presented by autologous cells (i.e. they were restricted). The restriction elements were probably at the level of HLA-DR antigens since the proliferative response was specifically blocked by anti-HLA-DR antibodies. Surface marker analysis revealed that the lines comprised mainly cells with an helper/inducer phenotype, although cells with markers of the suppressor/cytotoxic T cells were also present. The lines could be cloned by limiting dilution and clones with the same restriction and specificity as the parental line were isolated. These studies demonstrate the possibility of obtaining a large number of allergen specific human T cells that can be used for further in vitro studies on the regulation of the IgE response.