Protein-reactive chemicals and drugs (haptens), such as dinitro- (DNP) or trinitro-phenylated (TNP) reagents or penicillins, are potent inducers of allergic reactions in experimental animal systems as well as in humans. Hapten-specific T lymphocytes are mediators of these disorders. Like other antigens, hapten determinants on target cells are recognized by the T cell’s antigen-specific receptors exclusively in association with gene products of the major histocompatibility complex (MHC). Until recently, the nature of the complex hapten/MHC determinants remained unknown. Atypically, this fact, rather than spurring scientific investigations, caused a rapid loss of interest in haptens as T cell antigens by the majority of immunologists. This situation may now be changing, since we have recently identified MHC-binding, haptenmodified peptides which mediate cellular recognition by T cells via their antigen-specific receptors. Synthetic hapten-peptide conjugates open up new routes in studying the molecular details of hapten recognition by T cells. They may also contribute to a better understanding of what defines an antigen as an allergy-inducing “allergen”.