Abstract: There is described a method for protecting and conditioning a zone, such as an operating field, from a space. Said zone is bounded relative to the environment on at least one side thereof, by a curtain which is comprised of a substantially laminar stream from an essentially sterile gas such as air, and within said zone is fed preferably substantially continuously, some essentially sterile conditioned gas, in such a location, with such a speed and along such a direction that said curtain will not be disturbed.There is also described a device for the working of this method and a method for sterilizing a gas stream.