To clarify the different anti-CD26 mAbs corresponding different functions of CD26, the correlation of the epitopes defined by anti-CD26 mAbs and the functions of CD26 have been studied. Using truncated, human-rat CD26 swap mutants and cross-blocking studies, 13 anti-CD26 mAbs were divided into 5 separate groups. These 5 epitopes were localized between the 1-247th, 248-358th, 359-449th (closer to the 359th amino acid), 450-577th and 359 653th amino acid regions. MAbs against two of these five epitopes, the 248-358th and 359-449th amino acid regions, were associated with inducing modulation of CD26 and T-cell costimulation through the CD3 pathway. Furthermore, mAbs against one of these epitopes, the 359-449th amino acid region, appeared to encompass the ADA binding domain. Analysing the avidity of each mAb to the CD26 molecule using DPPIV enzymatic activity as an indicator, we found that the function of CD26 had little correlation with the avidity of anti-CD26 mAbs, suggesting that distinct epitopes defined by anti-CD26 mAbs appeared to be associated with different functions of CD26. These results will be very useful in the further definition of the functional domains of CD26.