These are a custom-made, patented spawn incubation bag with a microporous filter patch that allows gas exchange but precludes the passage of contaminants down to .3 microns. Made of 2.25 mil polypropylene, measuring 22.5 x 8.25 x 4.75 inches. These bags are larger than most spawn bags, allowing them to hold more material. Widely used by mushroom growers throughout the world.