The molecular analysis of thymopentin (TP5)/class II major histocompatibility complex (MHC II) complexes has been basically understood; however, the mechanism by which TP5-MHC II complexes are formed is largely unexplored. Compared with Epstein-Barr virus (EBV)-transformed B cells expressing human leucocyte antigen DR (HLA-DR), no fluorescent signal was observed on the DR-deficient cell line. This indicates that FITC labeled TP5 (FITC-TP5) is genuinely bound to HLA-DR. The binding specificity was confirmed by incubating FITC-TP5 With Unlabeled TP5 and HA peptide as well as mAb for DR molecules. In addition, the binding appeared to be rapid in living antigen-presenting cell (APC), which implies that TP5 is demonstrated on APC surface and does not require processing before associating with DR. Additional support for this SLII-face binding arises from the observation that pretreatment of cells with I variety of metabolic inhibitors failed to decrease the level of TP5/DR complexes. However, ternperatUre has an effect on the rate of binding between TP5 and DR molecules, which is well consistent with the qualitative predication of transition state theory. The formation of antigenic complexes is accelerated at acidic pH, which shows that the formation of TP5/DR complexes is a pH-dependent process.