Using a dinitrophenylated and biotinylated peptide antigen, we have developed an affinity chromatography procedure to purify complexes of a given peptide species and a given class II major histocompatibility complex antigen away from class II molecules occupied by other peptides. We show that hen egg lysozyme peptide-I-Ed complexes purified according to this procedure have a greatly enhanced capacity to activate hen egg lysozyme-specific T cells but have lost the capacity to activate three different alloreactive T-cell hybridomas. These data demonstrate that the class II molecule in and of itself is not sufficient to activate alloreactive T cells. Rather, the data suggest that recognition of specific complexes formed between allo-class II and particular autologous peptides may be required. Alternatively, alloreactive T cells may be recognizing "empty" major histocompatibility complex molecules.