The electrochemical protein-ligand assays based on the changes in the electrode response of the ligand labeled with electroactive compound were developed. The specific interaction between protein and the labelled ligand was detected by monitoring the changes in electrode response caused by the formation of a complex. The detection of the ligand was achieved by competitive reaction between the labeled ligand and unlabeled ligand for limited specific binding site of protein. Avidin-biotin and lectin-sugar systems were adopted as model protein-ligand. This method has an advantage since it does not require separation process of free labeled ligand from the bound one. In order to achieve electrochemical evaluation of the protein- ligand interaction, some ligands containing an electroactive group such as daunomycin, Nile Blue A, a complexing agent and a compound having functional as an electron-transfer mediator were prepared. The function introduced to biotin was controlled by avidin-biotin interaction. Furthermore, the interaction between protein and its ligand as self-assembled monolayer (SAM) on a gold electrode surface was electrochemically evaluated by monitoring the changes in the electrode response of the redox-marker ions. It was clear that the permeability changes in avidin membrane were ascribed to electrostatic interaction between avidin and marker ions.