A utility knife has a receiving cavity which opens through the body. A cartridge comprising a magazine of blades is removably insertable into the receiving cavity. A carrier within the body is manually operable for receiving a blade from the cartridge and positioning the blade for extension through the opening to present a cutting edge. The cartridge may be released from the utility knife body by depressing a button and withdrawing the cartridge. The cartridge is releasably held in place within the knife body and releasable when the blade carrier is in its retracted position and the release button is depressed. When the thumb button moves the blade carrier into its extended position to expose the knife blade, the cartridge is rendered unreleaseable even if the release button is depressed. The release button, which will enable removal of the magazine when depressed and blade carrier is in its retracted position, also serves to release of the blade from the blade carrier to permit removal of a used blade from the knife body when the blade carrier is in its extended position.