Therapeutically valuable proteins are often rare and/or unstable in their natural context, calling for production solutions in heterologous systems. A relevant example is that of the stress-induced, normally rare, and naturally unstable "read-through" human acetylcholinesterase variant, AChE-R. AChE-R shares its active site with the synaptic AChE-S variant, which is the target of poisonous organophosphate anticholinesterase insecticides such as the parathion metabolite paraoxon. Inherent AChE-R overproduction under organophosphate intoxication confers both short-term protection (as a bioscavenger) and long-term neuromuscular damages (as a regulator). Here we report the purification, characterization, and testing of human, endoplasmic reticulum-retained AChE-R_ER produced from plant-optimized cDNA in Nicotiana benthamiana plants. AChE-R_ER purified to homogeneity showed indistinguishable biochemical properties, with IC₅₀ = 10^-7 M for the organophosphate paraoxon, similar to mammalian cell culture-derived AChE. In vivo titration showed dose-dependent protection by intravenously injected AChE-R_ER of FVB/N male mice challenged with a lethal dose of paraoxon, with complete elimination of short-term clinical symptoms at near molar equivalence. By 10 days postexposure, AChE-R prophylaxis markedly limited postexposure increases in plasma murine AChE-R levels while minimizing the organophosphate-induced neuromuscular junction dismorphology. Our findings present plant-produced AChE-R_ER as a bimodal agent, conferring both short- and long-term protection from organophosphate intoxication.--Evron, T., Geyer, B. C., Cherni, I., Muralidharan, M., Kilbourne, J., Fletcher, S. P., Soreq, H., Mor, T. S. Plant-derived human acetylcholinesterase-R provides protection from lethal organophosphate poisoning and its chronic aftermath.