Bleach, but put in the deep freeze for a few days...it mysteriously whitens. 😁
Debra...If bleach won't work I might suggest adding lemon juice to your detergent.
Put them in the washer with hot water and bleach, let them soak and then dry them outdoors in the sun. While they are still wet from the washer smooth them into shape with your hands so they keep their shape and do not shrink.
Skip the bleach. Over time it will "yellow" or "gray" the fabric and weakens the fibers of the garment. I use baking soda, some use hydrogen peroxide. Here are other methods you might consider.