Several unanswered questions in T cell immunobiology relating to intracellular processing or in vivo antigen presentation could be approached if convenient, specific, and sensitive reagents were available for detecting the peptide-major histocompatibility complex (MHC) class I or class II ligands recognized by alphabeta T cell receptors. For this reason, we have developed a method using homogeneously loaded peptide-MHC class II complexes to generate and select specific mAb reactive with these structures using hen egg lysozyme (HEL) and I-Ak as a model system. mAbs specific for either HEL-(46-61)-Ak or HEL-(116-129)-Ak have been isolated. They cross-react with a small subset of I-Ak molecules loaded with self peptides but can nonetheless be used for flow cytometry, immunoprecipitation, Western blotting, and intracellular immunofluorescence to detect specific HEL peptide-MHC class II complexes formed by either peptide exposure or natural processing of native HEL. An example of the utility of these reagents is provided herein by using one of the anti-HEL-(46-61)-Ak specific mAbs to visualize intracellular compartments where I-Ak is loaded with HEL-derived peptides early after antigen administration. Other uses, especially for in vivo tracking of specific ligand-bearing antigen-presenting cells, are discussed.