A protein field-effect transistor (Pro-FET) based on the blue-copper protein azurins (see Figure) and operating at room temperature and ambient pressure is demonstrated. The transfer characteristics of the Pro-FET exhibit a pronounced resonance due to the switch from behaving as a n-metal oxide semiconductor FET (n-MOSFET) to a p-MOSFET. Carrier transport through the device is explained in terms of an equilibrium between the two possible oxidation states of the redox site (Cu1+ and Cu2+).