CD26 antigen, a 110 kDa membrane glycoprotein with exopeptidase activity (DAP IV), is an activation marker of T lymphocytes preferentially expressed on CD4+ memory cells and involved in T cell proliferation and IL-2 production after antigenic stimulation. We employed cytochemical and immunocytochemical techniques to study DAP IV/CD26 expression in circulating lymphocytes from 40 hemophilic patients, chronically treated with coagulation factors, in order to verify the possible involvement of this molecule in the immunological alterations of hemophilia. In all the hemophiliacs DAP IV activity was significantly lower than in the controls, independently of the quantity of blood transfused and previous exposure to viruses. This reduction may be responsible for the impaired proliferative response of lymphocytes to antigens and mitogens, notoriously observed in hemophilia. Whereas in the group of HIV- patients CD26 expression was similar to that of normal controls, in the 8 HIV+ hemophilic patients both percentages of positive lymphocytes and intensity of staining were significantly lower. In only 4 of the 8 cases was this deficit associated with CD4+ cell depletion. The significant selective loss of CD26 expression observed in HIV+ patients is probably an early event after HIV infection and seems to occur even before CD4 cell depletion. In conclusion, evaluation of DAP IV/CD26 might be a useful option for monitoring the immunological alterations of all hemophilic patients, HIV positive or not, chronically treated with coagulation factors.