Abstract: A retractable blade knife having a self-sharpening facility which can be selectively rendered inoperable. The sharpener is spring biased in an upward direction so as to be urged into engagement with the cutting edge of a knife blade, and manually operable release means is provided to permit the sharpener to be held in an inoperable position. An assembly of which the sharpener forms part also functions as stop means which positively prevents movement of the knife blade into its extended position. Such extension is possible only upon operation of the release means, but that is only necessary during initial outward movement of the knife blade. The sharpener is arranged to sharpen the cutting edge of the blade up to the tip of the blade. Positive locking means is provided to prevent inward movement of the blade when working under load in the extended position.