Aminopeptidase N (APN/CD13), a Zn2+-dependent ectopeptidase, is localized on the cell surface and functions as a transmembrane protein. Increased expression and activity of APN have been postulated to correlate with the aggressive behavior of several tumor types. In this study, the osteosarcoma cell line MNNG/HOS was stably transfected with an expression vector capable of expressing the antisense transcript of APN. Four stably transfected clones, the control clones and parental cells were characterized. Stable integration of the antisense vector was confirmed by PCR analysis of genomic DNA. Competitive RT-PCR revealed that mRNA expression of antisense-transfectants was decreased to approximately 37% of the control cell line. The activity assay showed that the enzymatic activity of APN was inhibited to approximately 51% of the control cell line. Antisense-transfection had no influence on the cellular proliferation measured by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, on the motility in Transwell chambers, and on the adhesive potential to collagen I. However, an in vitro invasion assay revealed a significant decrease in the number of cells that migrated through a reconstituted membrane (51% of the control cell line). The adhesive potential to Matrigel was also affected (73% of the control cell line). Furthermore, under in vivo conditions, a reduced potency to metastasize to the lung was shown in an experimental metastasis assay in nude mice. These findings demonstrate that APN plays an active role in the cellular attachment and proteolytic degradation of the extracellular matrix in the metastatic process of osteosarcomas.