The constant current chronopotentiometric reduction of five disulphide-containing proteins (bovine serum albumin, insulin, ribonuclease A, transferrin and trypsin), adsorbed on a hanging mercury drop electrode has been investigated. All five proteins showed a reduction peak at approximately -0.6 V vs. SCE due to the reduction of the disulphide groups. A linear relationship between protein molar mass and the adsorption area of the protein at monolayer coverage was observed, indicating that the proteins spread to a uniform thickness of approximately 10 Angstrom on the mercury surface.