The invention relates to a method of cleaning and/or sterilization of an object provided in a hermetically sealed enclosure, providing a pressure difference between an internal volume of the enclosure and surroundings and generating a plasma solely inside the enclosure for said cleaning and/or sterilization of the object. The invention further relates to an apparatus for enabling the same. The apparatus 10 comprises a vacuum chamber 1, which can be evacuated using a vacuum pump 2, and a source 3 arranged to generate plasma of a suitable gas in an enclosure 8, which is substantially hermetically closed with respect to the atmosphere of the vacuum chamber. The enclosure 8 may be of a flexible type or may be manufactured from a rigid material. In case when the enclosure is rigid the pressure inside the enclosure may be lower than an outside pressure.