Abstract: A surgical instrument comprised of a handle, a blade fixed to the forward end of the handle, and a spring-biased retractable sheath which is received within the handle. A mechanism is provided for retracting the sheath within the handle by depressing a single button on the upper surface of the handle, such that the sheath is retracted against the force of the spring as the button is depressed, and upon release of the button returns the sheath automatically to its released position covering the blade. Additionally, a mechanism is provided for resisting accidental exposure of the blade.