A sterilizer has an autoclave 1 within which is placed a sealed, non-porous bag 2 or box 50 containing an instrument 3 to be sterilized and a quantity of water or other liquid 22. When the temperature and pressure in the autoclave are raised, this raises the temperature and pressure of the container 2 and its contents sufficiently to effect sterilization, without any risk of contaminating the interior of the autoclave. The instrument 3 can be removed from the container and resterilized in the autoclave in the normal way, if desired.