The aim of this research was to determine the serum dipeptidyl peptidase IV (DPPIV) activity as well as the percentages of CD26 + lymphocytes and CD26 + overall white blood cells in patients with hematological malignancies: non-Hodgkin lymphoma (NHL), Hodgkin lymphoma (HL), leukemia, plasmacytoma and multiple myeloma, and in healthy individuals. Data from our study showed significantly decreased serum DPPIV activity and a significant decrease in the percentage of: CD26 + lymphocytes, CD26 + overall white blood cells and lymphocytes in patients with NHL in comparison to healthy controls. Patients with leukemia had a statistically significant lower activity of DPPIV in serum and significant decrease in the percentage of CD26 + lymphocytes in relation to healthy controls. Furthermore, significantly decreased DPPIV serum activity associated with a significantly reduced percentage of CD26 + overall white blood cells and percentage of lymphocytes was found in patients with multiple myeloma when compared to the healthy control group. The obtained results indicate that immune disturbances that can occur in hematological malignancies might be related to the decreased expression and activity of CD26/DPPIV that we observed.