This project aims to improve the diagnosis and therapy of head and neck cancer by use of radiolabeled monoclonal antibodies (MAbs). With respect to diagnosis: This part of the project focuses on the production of the long-living positron emitters Zirconium-89 [89Zr] and Iodine-124 [124I], their coupling to MAbs, and the in vivo evaluation of the radiolabeled MAbs for tumor detection with a PET camera. One of the possibilities explored to improve the efficacy of radioimmunotherapy, is by MAb engineering. To this end, the anti-CD44v6 murine MAb (mMAb) BIWA 1 was engineered to one chimeric version called BIWA 2 and two humanized versions called BIWA 4 and BIWA 8. Together with the anti-CD44v6 mMAb U36, directed against an overlapping epitope, these BIWA versions were evaluated for affinity to the antigen in vitro, as well as for biodistribution and efficacy in RIT using nude mice bearing the HNSCC xenograft line HNX-OE. As determined by surface plasmon resonance, the MAbs bound to CD44v6 with up to 46-fold difference in affinity (Kd ranging from 1.1x10-8 - 2.4x10-10 M) with the following ranking: mMAb U36 < hMAb BIWA 4 < hMAb BIWA 8 < mMAb BIWA 1 ~ cMAb BIWA 2. To evaluate their in vivo tumor-targeting properties, two MAbs with identical murine or human isotype were labeled with either 131I or 125I and administered simultaneously (50 µg/10µCi each) as pairs showing a stepwise decrease in the difference in affinity: Remarkably, for all three MAb pairs tested, the lower affinity MAb showed a higher degree and specificity of tumor localization. The difference in tumor localization was more pronounced when the difference in affinity was larger. Moreover, the same MAb pairs showed a RIT efficacy consistent with the biodistribution data. Based on these data we conclude that MAbs with markedly lower affinity to a given target antigen (e.g. U36, BIWA 4) may show superior tumor uptake and RIT efficacy in comparison with higher affinity versions of these MAbs.