Aim of this study was the extension of our knowledge on the occurrence and regulation of the membrane ectoenzyme aminopeptidase N (APN)/CD13 in various cell systems. Using immunofluorescence and flow cytometry, we examined APN expression on lymphocytes outside the peripheral blood. By use of flow cytometry, Ala-pNA cleavage and quantitative RT-PCR, we investigated the regulation of APN and colocalized membrane peptidases in various cells. Cell-cell contact as a reason of a lymphocytic APN expression was studied in coculture experiments of lymphocytes and adherent cells. APN positive lymphocytes were identified in different body fluids and tissues, among them synovial fluid of patients suffering from arthritis as well as in renal cell carcinoma. Inflammation-associated cytokines, such as interleukin (IL)-4 and interferon-γ, modulate APN and colocalized peptidases in different cells. IL-4 does not induce APN expression in pure lymphocytic cultures. However, for the first time we show that direct cell-cell contact of lymphocytes and synoviocytes results in a lymphocytic APN expression already after 60 min. Mitogens enhance and the cholesterol-depleting substance cyclodextrin abolishes lymphocytic APN expression after coculture. In summary, our results show that APN/CD13 is not a myeloid marker, since it is expressed on lymphocytes in special forms of inflammation. Inflammation-associated cytokines and cell-cell contact can regulate APN expression which is of influence on the cleavage of biologically active peptide mediators.