The binding of peptide antigens to class II histocompatibility glycoproteins can be markedly enhanced at pH values approximating those found in acidic endosomal compartments in antigen-presenting cells (APC). It has been proposed by others that low pH may increase the conformational flexibility of class II, facilitating both the association and dissociation of peptides. Neutralization of pH, as class II is expressed on the plasma membrane of APC, could then serve to trap peptide in a stable complex. If this were the only mechanism accounting for enhanced peptide binding at low pH, one would predict that there should be a concordance between the pH conditions required for enhanced binding and those associated with increased peptide dissociation. Furthermore, long-lived complexes of class II and peptide should not be observed at low pH without neutralization. In the present communization, I provide the data that support the generality of my previous conclusion that both affinity and maximal binding are regulated by pH in experiments using purified class II and biotin-labeled peptides. The pH profile for binding and dissociation using three different class II glycoproteins was analyzed, and the results demonstrated that enhanced binding is not coupled to enhanced dissociation. Peptide complexes were observed to be quite stable at pH 4.5 and above. This result was further substantiated in experiments where biotin-peptide/class II complexes were extensively dialyzed at low pH followed by analysis on Western blots probed with avidin. Finally, a low pH assay system was devised to analyze the formation of stable peptide/class II complexes without pH neutralization. Our results indicate that stable complexes can be formed at low pH without the requirement for a shift to neutral pH.