An enclosure (23) for carrying out operations under sterile conditions having apparatus for sterilsing the enclosure and its content includes a gas generator (17,18) having a fan (18) for producing a stream of air and an evaporator (17) for evaporating hydrogen peroxide solution to be carried by the airstream throughout the enclosure to condense on surfaces in the enclosure to sertilise the surfaces. A pump (13) is provided to draw gas from the enclosure at a location remote from the gas generator to ensure that the sterile vapour reaches the most remote part of the enclosure from the generator and also to maintain the pressure in the generator below atmospheric pressure around the generator so that any leak path in the enclosure results in leakage from the atmosphere into the enclosure and not release of sterilant vapour to atmosphere outside the enclosure.