© 2014, Academy of Molecular Imaging and Society for Molecular Imaging. Purpose: The use of receptor-targeted antibodies conjugated to fluorophores is actively being explored for real-time imaging of disease states; however, the toxicity of the bioconjugate has not been assessed in non-human primates.Procedures: To this end, the in vivo toxicity and pharmacokinetics of IRDye800 conjugated to cetuximab (cetuximab-IRDye800; 21 mg/kg; equivalent to 250 mg/m2 human dose) were assessed in male cynomolgus monkeys over 15 days following intravenous injection and compared with an unlabeled cetuximab-dosed control group.Results: Cetuximab-IRDye800 was well tolerated. There were no infusion reactions, adverse clinical signs, mortality, weight loss, or clinical histopathology findings. The plasma half-life for the cetuximab-IRDye800 and cetuximab groups was equivalent (2.5 days). The total recovered cetuximab-IRDye800 in all tissues at study termination was estimated to be 12 % of the total dose. Both cetuximab-IRDye800 and cetuximab groups showed increased QTc after dosing. The QTc for the cetuximab-dosed group returned to baseline by day 15, while the QTc of the cetuximab-IRDye800 remained elevated compared to baseline.Conclusion: IRDye800 in low molar ratios does not significantly impact cetuximab half-life or result in organ toxicity. These studies support careful cardiac monitoring (ECG) for human studies using fluorescent dyes.