The metal-dependent membrane alanyl aminopeptidase (amino-peptidase N, APN, CD13; EC 3.4.11.2) is a well-established marker of normal and malignant cells of the myelo-monocytic lineage. It is also expressed by leukaemic blasts of a small group of patients suffering from acute or chronic lymphoid leukaemia. CD13-specific monoclonal antibodies do not bind to the surface of normal B lymphocytes, and APN mRNA was not detectable by Northern analysis in normal lymphocytes or in T-cell lines. Recently the expression of the APN gene in T-cell lines as well as the ability of these cells to cleave chromogenic substrates preferred by APN have been demonstrated [Lendeckel, Wex, Kähne, Frank, Reinhold and Ansorge (1994) Cell. Immunol. 153, 214-226]. Here, by means of dot-blot hybridization and RNase protection assay, evidence is provided that human peripheral T-cells as well as derived cell lines contain significant amounts of APN mRNA, comparable to that in the promyeloic cell line U937, and that mitogenic activation of peripheral human T-cells leads to a more than 4-fold increase in their APN mRNA content. In the course of activation, T-cells increase their total alanine p-nitroanilide-hydrolysing activity to approx. 7-fold that of resting cells. Furthermore these cells become immunoreactive towards CD13 to a significant extent (up to 51%) as shown by surface staining and confirmed by activity staining and immunostaining after isoelectric focusing (pI of T-cell APN = 4.6). In addition it is demonstrated by fluorescence microscopy that viable, activated T-cells effectively cleave the fluorogenic aminopeptidase substrate bis-glycyl-rhodamine 110 and that the corresponding aminopeptidase activity is associated with the cell surface. We show that specific inhibitors of APN, probestin and actinonin, strongly decrease DNA synthesis in phytohaemagglutinin (PHA)-stimulated T-cells. In summary, evidence is presented that in the course of mitogenic activation human peripheral T-cells increase the expression of APN both at the transcriptional level and at the cell surface. This has been demonstrated both at the APN mRNA level and at the protein level with respect to aminopeptidase enzymic activity and CD13 immunoreactivity.