Gel filtration and gel electrophoresis of refolded HLA-B27/Chlamydia peptide monomers loaded with Chlamydia peptide 133 (a) and with Chlamydia peptide 8 (b). The peaks eluted at 13.7 ml contained the soluble MHC monomer. The amount of refolded HLA-B27 monomer with Chlamydia peptide 8 (b) was higher than that of peptide 133 (a) with less protein aggregation (proteins eluted between 7 and 13 ml), indicating that peptide 8 has a higher binding affinity for HLA-B27. The SDS-PAGE in (a) and (b) shows that both eluted monomers are the major protein in the eluted fraction and that most soluble monomers bind to streptavidin if added. In both experiments the amount of protein aggregation was higher than refolding of HLA-B27 with a viral epitope, indicated by a greater amount of eluted proteins between 7 and 13 ml (Fig. 1). A280, absorption at 280 nm; asterisks, protein aggregates; SA, streptavidin.