Glioblastoma multiforme (GBM) is the most aggressive form of primary brain tumor type and is associated with a high mortality rate borne out of such affording a survival rate of only 15 months. GBM aggressiveness is associated with the overexpression of epidermal growth factor receptor (EGFR) and its mutants. Targeting GBM with therapeutics is challenging because the blood-brain barrier (BBB) permits primarily select small-molecule entities across its semipermeable blockade. However, recent preclinical data suggest that large biomolecules, such as the anti-EGFR antibody therapeutic, cetuximab, could be capable of bypassing the BBB despite the relative enormity of its size. As such, we set forth to establish the feasibility of utilizing an EGFR-targeting near-infrared-I (NIR-I) fluorescent construct in the form of an immunoconjugate (cetuxmimab-IRDye800) to achieve visual differentiation between diseased brain tissue arising from a low-passage patient-derived GBM cell line (GBM39) and healthy brain tissue via utilizing orthotopic intracranial murine GBM39 tumor models for in vivo and ex vivo evaluation such that by doing so would establish proof of concept for ultimately facilitating its in vivo fluorescence-guided resection and ex vivo surgical back-table pathological confirmation in the clinic. As anticipated, we were not capable of distinguishing between malignant tumor tissue and healthy tissue in resected intact and slices of whole brain ex vivo under white-light illumination (WLI) due to both the diseased tissue and healthy tissue appearing virtually identical to the unaided eye. However, we readily observed over an average 6-fold enhancement in the fluorescence emission in the resected intact whole brain ex vivo when performing NIR-I fluorescence imaging (FLI) on the cohort of GBM39 tumor models that were administered the immunoconjugate compared to controls. In all, we laid the initial groundwork for establishing that NIR-I fluorescent immunoconjugates (theranostics) such as cetuximab–IRDye800 can bypass the BBB to visually afford GBM39 tumor tissue differentiation for its image-guided surgical removal.