In the human T-cellline H9 a neutral aminopeptidase activity was detected that appeared to be distinguished from other known aminopeptidases with respect to its biochemical characterisation. The aim of this work was to identify this enzyme and to subject it to comprehensive biochemical characterization. The enzyme was purified to homogeneity by the application of a number of conventional chromatographic methods and identified by means of mass spectrometry (Maldi TOF) as cytosol alanyl aminopeptidase (EC 18.104.22.168; cAAP, PSA). The biochemical characteristics of the H9 cell-derived peptidase revealed similarities to the corresponding enzyme of other tissues, however, substantial differences have been found out. Apparently, due to these tissue-specific differences there have been assigned different names to this enzyme before. The cytosolic alanyl aminopeptidase showed high sensitivity to inhibitors that have been regarded as specific for the membrane-bound aminopeptidase N (APN): Phebestin, probestin or RB3014 effectively inhibited the cytosol alanyl aminopeptidase as well, which could be explained by a high degree of similarity of the catalytic domains of both peptidases. In addition, "APN-specific" monoclonal antibodies that are widely used for leukemia typing exhibited strong cross-reactivity in those cases where the epitope recognized is near the catalytic site. The data presented show that the expression in peripheral T-cells of the cytosol alanyl aminopeptidase could be induced by the administration of mitogens such as PHA or PWM. This activation-dependent increase of the expression was determined at the mRNA level using of the quantitative RT-PCR technique. This activation-dependent increase in cAAP mRNA expression could be modified by the simultaneous administration of aminopeptidase inhibitors, an effect that greatly dependent on the T-cell stimulants applied. In summary, the data presented make a contribution to an better understanding of the biochemical characteristics, the regulation of expression, and the physiological and pathophysiological role of the H9 cell derived cytosol alanyl aminopeptidase. The results may pace the way for future applications of aminopeptidas inhibitors as anti-inflammatory drugs.