CD26/dipeptydil peptidase IV (DPPIV) is a surface glycoprotein that is expressed on a variety of tissues, comprising T and B cells. It has an intrinsic dipeptidyl peptidase IV activity, which cleaves NH2- terminal dipeptides from polypeptides with either L-proline or L-alanine at the penultimate position. It plays a role in immune regulation transmitting T cell activation signals. It also seems to play an important role in the mobilization of hematopoietic stem cells and hematopoietic progenitor cells. Natural substrates of CD26 contain the N-terminal amino acid sequence with proline in the second position, and include many cytokines and growth factors, many of them involved in hematopoiesis. Recent investigations have suggested an important role for CD26 in the development of certain human cancers, including hematological malignancies. The aim of our project is to examine surface CD26 expression in peripheral blood samples of patients with T-ALL (T-acute lymphoblastic leukemia), B-ALL (B-acute lymphoblastic leukemia) and AML (acute myeloid leukemia) comparing with healthy subjects. Ten untreated patients were included in this study. The patient age ranged from 5 to 19 years. Their cells were characterized for the presence of CD26 by flow cytometric analysis at The National Institute of Cancer, Rio de Janeiro, Brazil. Cases showing more than 20% of mononuclear cells reactive with antibody against CD26 were considered positive for CD26 protein. Reverse transcriptase polymerase chain reaction analysis showed the transcript levels of CD26. Surface CD26 expression was concordant with the presence of CD26 transcripts in four cases. In addition, in three cases, no cell surface protein was found, while no CD26 mRNA was detected. However, we detected CD26 mRNA, but no cell surface CD26 expression, in three cases, suggesting that, despite the absence of cell surface protein, CD26 was expressed at the transcriptional level. Results obtained from healthy subjects are being investigated, in order to compare the pattern of CD26 expression between leukemic and normal cells. It is known that homeostatic regulation of hematopoiesis involves temporally controlled expression of cytokines and their receptors. Therefore, if we have found a statistically significant difference between the pattern of CD26 expression in normal and leukemic cells, it is possible that altered levels of CD26 expression be a relevant factor for malign transformation in the hematopoietic system.