Based on choline oxidase immobilized by co-crosslinking on an overoxidised polypyrrole modified platinum electrode, a novel electrochemical assay for cholinesterase activity in human serum was developed. The assay was performed by adding an aliquot of cholinesterase standard solution or serum sample to phosphate buffer containing choline or thiocholine ester and measuring the oxidation current of hydrogen peroxide at the rotating modified electrode polarized at +0.7 V vs. SCE. The influence of some experimental parameters such as pH of the assay, mass transport at the electrode, type and concentration of the cholinesterase substrate was studied and optimised. Reversible inhibition of choline oxidase from cholinesterase substrates was evidenced for the first time, which increases in the order of acetylcholine, butyrylcholine and s-butyrylthiocholine. Wide linear range, fast response time and appreciable long-term stability were assured for both acethyl- and butyrylcholinesterase assays. On allowing the polypyrrole layer to efficiently remove interferences from the electroactive compounds in the sample, the present method revealed to be suitable for the detection of butyrylcholinesterase in human serum at activities as low as 0.5 U L-1. The validation with a reference spectrophotometric method showed no significant differences when human serum samples were analysed.