Label-free electrical detection of protein interactions has been achieved by direct measurement of variations in open circuit potential (OCP) using an accurate differential voltage measurement. Our model system involves a panel of peptide aptamers that recognise specific protein partners of the cyclin-dependent kinase (CDK) family. Different peptide aptamers immobilized on gold electrodes were used for the detection of human CDK2 and CDK4. The formation of the peptide aptamer recognition layer and the efficiency of its interaction with CDK proteins in yeast cell lysates were characterized by quartz crystal microbalance measurements. The interaction of the peptide aptamers with CDK proteins was successfully detected by direct OCP measurements. The OCP dependence on the pH of the measurement buffer confirms that the effects observed are due to the change in charge upon protein interaction. Variations in charge transfer resistance and in protein/double-layer capacitance were investigated by means of electrochemical impedance spectroscopy with charged redox markers in solution. The present work shows that electrical detection of protein interactions can be achieved by direct measurement of OCP variations using suitable differential voltage instrumentation. The results indicate that label-free detection of protein interactions is possible with potentiometric transducers such as field-effect transistors.