Abstract: A process for forming plastic tubing which involves extruding a molten plastic composition through a die which produces an extended length of plastic tubing, passing the hot tubing through a vacuum sizing tube wherein the wall of the tubing is sized by being drawn to the wall of the sizing tube by a vacuum, and cooling the sized tubing so that the tubing will be self-supporting at normal ambient temperatures, wherein the plastic composition contains a phosphorus-containing additive in an amount sufficient to cause an improvement in the smoothness of the interior of said tubing.