A circulatory long-lived cholinesterase (ChE) protein, such as acetylcholinesterase (AChE) or butyrylcholinesterase (BChE), which is a ChE protein modified with a non-antigenic polymer. The ChE may be AChE, such as native AChE of mammalian origin or of non-mammalian origin, or recombinant AChE. The recombinant AChE may be mutated at one or more amino-acid residues. The BChE may be native BChE of mammalian origin or of non-mammalian origin.