Monoclonal antibodies (mAbs) are an emerging class of therapeutics for the prevention and treatment of viral infections. Recent advances in mRNA/lipid nanoparticle (LNP) technology provide a promising new modality for the production of mAbs, potentially bypassing the need for recombinant manufacturing of mAb proteins. In this study, we compared traditional infusion of protein-based neutralizing mAbs targeting severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) or influenza viruses to mRNA/LNP-based production of mAbs in treated mice. High serum concentrations of mAbs were achieved upon delivery of a single mRNA encoding both heavy and light chains via intravenous or intramuscular routes using prototypic LNP formulations. However, the pharmacokinetics of mRNA-delivered mAbs were heavily influenced by the induction of anti-drug antibody responses directed against the encoded mAbs, resulting in reduced half-lifeand compromised protective capacity against SARS-CoV-2 Omicron BA.1 infection. In contrast, mRNA/LNP delivery of a neutralizing mAb conferred superior protection against lethal influenza challenge compared to equivalent recombinant protein doses. Overall, mRNA/LNP delivery comprises a feasible and attractive pathway to speed the development and deployment of antiviral antibodies. However, optimization of LNP formulation, dosing, and administration routes is required to maximize protective potential. in vivo in vivo