We have electrosynthesized a hydrophilic biotinylated polypyrrole film and studied it by gravimetric measurements using a quartz crystal microbalance (QCM). We demonstrated that the quantity of immobilized avidin is a function of the number and the accessibility of biotin units in the polypyrrole film. From QCM results, it appears that the amount of anchored avidin could be modulated by the quantity of biotin units inserted in the polypyrrole matrix and also by the spacer arm length linking biotin units to the polypyrrole network, For this last case, the coverage rate in avidin could vary from 8 to 16 pmol cm (2).