In this study we investigated the binding or three different monoclonal antibodies (MoAb), TII 19-4-7,4EL1C7, and B1.19.2, which are clustered in CD26 to the ectoenzyme dipeptidyl peptidase IV (DPP IV) and to T lymphocytes. We found that all three MoAb bind to both unstimulated and mitogen-stimulated T lymphocytes. Further results indicated an inconsistency within the CD26-clusteredMoAb: TII 19-4-7 and 4EL1C7, but not Bl.19.2, recognized DPP IV on the surface of T lymphocytes and immobilized on solid-phase ELISA or Western blot. There was compelition of binding to DPP IV between TII 19-4-7 and 4ELIC7. From these results we conclude that CD26 antigen is represented by the ectoenzyme DPP IV. TII 19-4-7 and 4ELIC7 recognize the same or partly identical epitopes on DPP IV, whereas B1.19.2 recognizes a different antigen. TI I 19-4-7 and 4ELlC7, but not B1.19.2, should be clustered in CD26.