The low expression of HER3 in malignancies and moderate expression in normal tissues41 mandate targeted drugs and imaging radiotracers to have a high avidity to this antigen40. Sub-nanomolar or higher affinities are preferable for improved tumor specificity and selectivity36,40. We examined whether the binding affinity of 89Zr-MEHD7945A for EGFR and HER3 were retained after radiolabeling. The results from the in vitro and ex vivo competitive binding assays exhibited nanomolar affinities, in good agreement with the established KD of the unmodified antibody17. Other immunoPET tracers singly targeting HER3 fall within the same range (i.e. 2.7 nM42 and 6.8 nM37). Only one other tracer, the affibody-based probe 68Ga-HEHEHE-Z08698-NOTA, exhibited picomolar binding affinity (50 pM)36,43. Established affinities of other immunoPET EGFR tracers are within the same range. For example, an affibody labeled with F-18 displayed a KD ~ 37 nM whereas a 67Ga-DOTA-cetuximab F(ab′)2 reported a KD ~ 8.6 nM, making the KD for our radiotracer within reasonable range.