The present treatment for poisoning by organophosphates consists of multiple drugs such as carbamates, antimuscarinics, and reactivators in pre- and post-exposure modalities. Recently an anticonvulsant, diazapam, has been included as a post-exposure drug to reduce convulsions and increase survival. Most regimens are effective in preventing lethality from organophosphate exposure but do not prevent toxic effects and incapacitation observed in animals and likely to occur in humans. Use of enzymes such as cholinesterases as pretreatment drugs for sequestration of highly toxic organophosphate anticholinesterases and alleviation of side effects and performance decrements was successful in animals, including non-human primates. Pretreatment of rhesus monkeys with fetal bovine serum acetylcholinesterase protected them against lethal effects of soman (up to 5 LD50) and prevented signs of OP toxicity. Monkeys pretreated with fetal bovine serum acetylcholinesterase were devoid of behavioral incapacitation after soman exposure, as measured by serial probe recognition or primate equilibrium platform performance tasks. Use of acetylcholinesterase as a single pretreatment drug provided greater protection against both lethal and behavioral effects of potent organophosphates than current multicomponent drug treatments that prevent neither signs of toxicity nor behavioral deficits. Although use of cholinesterases as single pretreatment drugs provided complete protection, its use for humans may be limited, since large quantities will be required, due to the approximately 1:1 stoichiometry between organophosphate and enzyme. Bisquaternary oximes, particularly HI-6, have been shown to reactivate organophosphate-inhibited acetylcholinesterase at a rapid rate. We explored the possibility that enzyme could be continually reactivated in animals pretreated with fetal bovine serum acetylcholinesterase, followed by an appropriate dose of reactivator, and challenged with repeated doses of sarin. In in vitro experiments, stoichiometry greater than 1:400 for enzyme:sarin was achieved; in vivo stoichiometry in mice was 1:65. Pretreatment of mice with fetal bovine serum acetylcholinesterase and HI-6 amplified the effectiveness of exogenous enzyme as a scavenger for organophosphate.