Polypeptides which have been separated by gel electrophoresis can be identified by electroblotting them through a "sandwich" comprising in order: a) the separation gel; b) at least one hydrophilic membrane, e.g. of carboxyl-modified PVDF, on which is immobilized at least one reagent capable of cleaving a polypeptide, e.g. trypsin; c) a hydrophobic layer, typically a membrane, e.g. of PVDF. Preferably a biased alternating current or discontinuous direct current is used for electroblotting. The resulting fragments, usually peptides, are identified, preferably by MALDI-TOF MS.