Extensive pharmacokinetic studies in both mice and rhesus macaques, with biochemically well defined forms of native and recombinant AChEs from bovine, rhesus and human origin, allowed us to determine an hierarchical pattern by which post-translation-related factors and specific amino-acid epitopes govern the pharmacokinetic performance of the enzyme molecule. In parallel, we demonstrated that controlled conjugation of polyethylene-glycol (PEG) side-chains to lysine residues of rHuAChE also results in the generation of active enzyme with improved pharmacokinetic performance. Here, we show that equally efficient extension of circulatory residence can be achieved by specific conditions of PEGylation, regardless of the post-translation-modification state of the enzyme. The masking effect of PEGylation, which is responsible for extending circulatory lifetime, also contributes to the elimination of immunological responses following repeated administration of AChE. Finally, in vivo protection studies in mice allowed us to determine that the PEGylated AChE protects the animal from a high lethal dose (2.5 LD(50)) of soman. On a mole basis, both the recombinant AChE and its PEGylated form provide higher levels of protection against soman poisoning than the native serum-derived HuBChE. The findings that circulatory long-lived PEGylated AChE can confer superior protection to mice against OP-compound poisoning while exhibiting reduced immunogenicity, suggest that this chemically modified version of rHuAChE may serve as a highly effective bioscavenger for prophylactic treatment against OP-poisoning.