Immoblhsatlon of acetylcholme esterase (ACE) and cholme ondase (COx) Hnthm a polymenc coatmg on a glassy carbon electrode resulted m the development of a sensor for the unportant transrmtter acetylchohne and Its metabohte choline Anodlc detection at +0 25 V vs SCE 1s facdltated by the addltlon of the redox-medlatmg hexacyanoferrate(II1) Ion The influence of vanous experunental parameters IS described Lmeanty for acetylchohne and chohne responses extended up to 5 mM and 13 mM, with llrmts of detectlon of 43 PM and 50 PM, respectively The simple unmoblhsation procedure and the rapld reactlon kmetlcs between the mediator and the enzyme, coupled to a fast heterogeneous rate constant, resulted m rapid response tmes and good preclslon An unportant apphcatlon of the blosensor was shown for the detenmnatlon of arsenochohne, usmg the chohne oxldase munobtid electrode, resultmg m a response ten times that obtamed for chohne, with hneanty extendmg up to 0 6 mM and a lmut of detection of 10 WM The Hugh selectlvlty offered by tlus sensor was indicated by the lack of mterference offered by easily oxldlsed compounds normally found m serum, because of both the charge exclusion properties of the Nafion polymenc coating and the low operatmg potential required to re-ondlse the mediator