Abstract: A protective sheath for a disposable surgical scalpel includes at least three modes of operation, (i) a protective mode, (ii) an open mode, and (iii) a permanently locked mode. In the open mode, a locking strip of the scalpel handle resides in one opening of the sheath to keep the sheath substantially immobile during use of the scalpel blade. In the protective mode, the locking strip resides in a second opening of the sheath to keep the sheath substantially immobile when the scalpel blade is not in use. In the permanently locked mode, a button connected to the sheath is depressed into the second opening and engages the locking strip, thereby preventing the blade from being used and allowing the scalpel to be safely disposed.