More than 60% of anaplastic large‐cell lymphomas (Ki‐1 lymphoma) are associated with a t(2;5)(p23;q35) translocation that produces an 80 kDa hyperphosphorylated chimeric protein (p80) derived from the fusion of the anaplastic lymphoma kinase (ALK) with nucleophosmin (NPM). The NPM‐ALK chimeric gene is an activated tyrosine kinase that has been shown to be a potent oncogene. We have developed a cellular model for the study of p80 action in rat la fibroblasts. Expression of cDNA's encoding NPM‐ALK (p80) in rat la fibroblasts induces anchorage‐independent growth in soft agar and promotes foci formation in culture. Cells expressing exogenous p80 showed significantly increased proliferation characterized by accelerated cell cycle entry into S‐phase. Consistent with increased G₀/G₁ to S‐phase transition, there is also marked up‐regulation of cyclin A and cyclin D1 expression. In addition, p80 transformed cells showed elevated expression of several immediate early genes involved in cellular proliferation, including fos, jun, and c‐myc. DNA binding analysis of nuclear extracts prepared from p80 transformed cells reveal marked up‐regulation of AP‐1 DNA binding activity. Functional AP‐1‐specific transfection assays also show up‐regulation of AP‐1‐dependent transcriptional activation. These finding demonstrate that p80 transformed rat la fibroblast can be a highly useful model system for the molecular and biochemical characterization of the mechanisms of action of this interesting new oncogene.—Wellmann, A., Doseeva, V., Butscher, W., Raffeld, M., Fukushima, P., Stetler‐Stevenson, M., Gardner, K. The activated anaplastic lymphoma kinase increases cellular proliferation and oncogene up‐regulation in rat la fibroblasts. FASEB J. 11, 965–972 (1997)