This paper considers an amperometric biosensor functioning in the following manner : an oxidase enzyme reacts with its substrate to produce H2O2, which is then reduced to water at a conducting polymer film containing uniformly distributed microparticulate metal centres. The situations of the enzyme in bulk solution and co-immobilised within the polymer film, are both considered. For both cases, approximate analytical expressions are derived which relate the response of the sensor to experimentally controllable parameters, such as substrate concentration, film thickness, enzyme loading, etc.