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The preparation of a cholesterol amperometric biosensor using a platinized Pt electrode as a support for the electropolymerization of a polypyrrole film, in which cholesterol oxidase and ferrocene monocarboxylic acid (electron-transfer mediator) were co-entrapped, is described. All the biosensor preparation steps (platinization and electropolymerization) and the cholesterol determination take place in the same flow system. The presence of the mediator enhances the sensitivity and selectivity of the platinized biosensor without modifying the dynamic parameters of the response, and the platinized layer improves the operational lifetime of the mediated sensor. The sensitivity obtained was 88.51 nA mM(-1) and the limit of detection was 12.4 microM of cholesterol. The analytical properties of the biosensor for the flow-injection determination of cholesterol were studied and compared with those of other more simple amperometric biosensor configurations.