Generation of an HLA B27/beta 2 microglobulin/peptide tetramer is shown in cartoon form. Denatured class 1 heavy chain is refolded by limiting dilution in the presence of beta 2-microglobulin and excess peptide. After chromatographic purification, correctly folded complexes are biotinylated as shown. Four of these complexes are bound to fluorochrome-conjugated streptavidin (as shown in Fig. 1) for use in FACS staining of T cells.