The invention relates to a sterile system and a method for filtering biological or medical liquids, especially blood or blood constituents. According to the filtering system and method, the sterile air already in the system is collected before the beginning of the filtration process, and after the filtration process, in order to completely empty the system used. Said filtering system comprises a collapsible container having a first and second air inlet/outlet line. The first air inlet/outlet line communicates with the first chamber of the filter by means of a flow connection, and the second air inlet/outlet line with the second chamber of the filter. Only the air inlet/outlet line communicating with the first filter chamber is sealed by a hydrophobic membrane or a return valve in the filtering system, such that the system has an especially simple structure.