Abstract: A retractable, disposable surgical knife that includes a hollow sheath and a blade assembly received for sliding movement within the hollow sheath. The blade assembly includes a slide block and a knife blade held by the slide block. A mechanism including a cooperating resilient actuator and guide track allows the blade assembly to be selectively secured in a first, retracted position wherein the blade is withdrawn into the sheath and a second, extended position wherein the blade extends from the sheath for use. A separate mechanism is provided for permanently locking the blade assembly in a third, fully retracted position wherein blade is fully withdrawn within the sheath to prevent inadvertent sticks. The permanent locking mechanism is fully concealed within the sheath so as to prevent inadvertent engagement with an object and possible release from the permanently locked position.