Nimotuzumab is a humanized monoclonal antibody against the Epidermal Growth Factor Receptor with a long history of therapeutic use, recognizing an epitope different from the ones targeted by other antibodies against the same antigen. It is also distinguished by much less toxicity resulting in a better safety profile, which has been attributed to its lower affinity compared to these other antibodies. Nevertheless, the ideal affinity window for optimizing the balance between anti-tumor activity and toxic effects has not been determined. In the current work, the paratope of the phagedisplayed nimotuzumab Fab fragment was evolved in vitro to obtain affinity-matured variants. Softrandomization of heavy chain variable region CDRs and phage selection resulted in mutated variants with improved binding ability. Two recombinant antibodies were constructed using these variable regions, which kept the original fine epitope specificity and showed moderate affinity increases against the target (3-4-fold). Such differences were translated into a greatly enhanced inhibitory capacity upon ligand-induced receptor phosphorylation on tumor cells. The new antibodies, named K4 and K5, are valuable tools to explore the role of affinity in nimotuzumab biological properties, and could be used for applications requiring a fine-tuning of the balance between binding to tumor cells and healthy tissues.The epidermal growth factor receptor (EGF-R) remains one of the best-established targets for anti-tumor therapies. This receptor is involved in cellular processes that contribute to the survival of epithelial cells. Deregulation of the EGF/EGF-R pathway by receptor overexpression or constitutive activation promotes tumor cell proliferation, invasion, and is associated with poor prognosis in cancer 1 . Up to now three anti-EGF-R monoclonal antibodies (mAbs) have been approved for clinical use by the FDA: cetuximab (2004), panitumumab (2006) and necitumumab (2015) 2 . Nimotuzumab recognizes the same target and has a long history of therapeutic use 3 , starting with clinical trials since 1998, and first registered by the Cuban regulatory authorities in 2002 4 . Nimotuzumab is currently approved in Cuba for the treatment of childhood and adult glioma, advanced esophageal cancer, and squamous cell carcinoma of the head and neck, in combination with chemo-radiotherapy or radiotherapy alone, and is also registered in 28 additional countries.The availability of several mAbs targeting the same tumor antigen can result in different clinical outcomes in terms of therapeutic efficacy, safety, anti-tumor mechanisms, immunogenicity, patients' sub-population that can receive a benefit, and development of resistance to therapy. The major molecular determinants behind such complex landscape of clinical effects are the origin of constant domains (species and isotype), the strength of binding to the target (affinity) and the topology of interaction with the specific antigen region that is recognized (epitope specificity). Properties associated to the nature of co...