Abstract: A cloth bag filter is formed with a generally cylindrical porous sidewall through which a vacuum is drawn for filtering of contaminant from an air stream, an open upper end for connection to the vacuum source, and a generally closed but optionally porous lower end portion. The sidewall is sufficiently flexible such that it collapses radially inwardly when air is flowing therethrough, simultaneously causing the lower end portion of the bag to be drawn upwardly toward the upper end portion. Upper and lower rings are secured to the upper and lower end portions of the bag for radially supporting the end portions. The weight of the lower ring causes the lower portion of the bag to automatically drop when the vacuum source is turned off, thus causing the sidewall to automatically expand outwardly to a generally vertically hanging position, disrupting the cohesion of the contaminant collected thereon, and causing such contaminant to break-up and fall from the filter.