Background IFN alpha is an important cytokine of the immune system. It has the ability to interfere with virus replication exerting antiviral activity. Moreover, it displays antiproliferative activity and can profoundly modulate the immune response. IFN4N (or hyperglycosylated IFN alpha) is an IFN-alpha2b mutein developed in our laboratory using glycoengineering strategies. This molecule contains 4 potential N-glycosylation sites together with the natural O-glycosylation site in Thr106 [1]. The resulting Nand O-glycosylated protein shows higher apparent molecular mass and longer plasmatic half-life compared to the nonglycosylated IFN-alpha produced in bacterial systems and used for clinical applications. As a consequence, the correct glycosylation of our modified cytokine is very important for its in vivo activity. For this reason, it is of great relevance the evaluation of different mammalian host cells for its production. While hamster-derived CHO cells are widely used for large scale production of recombinant therapeutic glycoproteins, human HEK cells are a promising system because they are easy to grow and transfect [2]. In this work, we performed a comparison between both production systems in terms of cell growth, culture parameters and specific productivity of hyperglycosylated IFN alpha.