A reusable filter port assembly is installable on the wall of a sterilizable cultivation bag, such as is used in the cultivation of mushrooms. A grommet and locking collar are removably installed in the wall of the cultivation bag or pouch to provide gas flow communication between the interior of the bag and the ambient environment after the bag is sealed. A removable filter cap holds a filter member over the opening of the filter port assembly to prevent undesired particles from passing through the port of the assembly. In use, the grommet is placed against the interior surface of a bag or pouch, such as a plastic cultivation bag. The collar is screwed down over the grommet from the exterior surface of the bag to form a grommet-collar combination fixed in place to the wall of the bag. Once installed, a patch of bag material of the bag wall is still covering the grommet opening of the neck tube. The patch of bag material is then cut out or otherwise removed from the port opening to provide communication between the interior of the bag and the ambient environment. The filter cap is attached to the grommet-collar combination to generally allow the passage of gasses and prevent passage of particulates.