Abstract Fel d I, the major cat dander allergen, is recognized by serum IgE of more than 80% of all cat-allergic patients. Because IgE synthesis by B lymphocytes is under the control of T lymphocytes, we studied the specificity and lymphokine production profiles of cat dander-specific T lymphocytes. Polyclonal cat dander-specific T cell lines were found to react with purified Fel d I, but not with cat albumin, the only other characterized cat allergen. Similarly, within a panel of CD4+ T lymphocyte clones (TLC) that was generated from these cat dander-specific T cell lines, 5 of 16 TLC were found to react with Fel d I, and 0 of 16 with cat albumin. The remaining 11 TLC were shown to recognize at least two different proteins. In general, the TLC had a high IL-4/IFN-gamma production ratio, and could recognize the cat dander extract in an HLA-DR, HLA-DQ, or HLA-DP restricted manner. In addition, five distinct T cell epitopes of Fel d I were identified by using a panel of overlapping synthetic peptides of both chains of Fel d I. The data presented here indicate that, even though multiple proteins in cat dander extract are recognized by T lymphocytes of allergic patients, Fel d I, the major IgE binding allergen, is also important in T cell activation. The fact that the cat-specific TLC are Th2-like indicates that these cells may play an important role in the pathophysiology of allergic responses to cat allergens. However, the diversity of HLA-class II restriction of cat dander- and Fel d I-specific TLC and the presence of multiple T cell epitopes in the allergen may complicate future immunotherapies.