I would soak it in Oxyclean and hot water (or at least warm water) according to the directions on the Oxyclean container.

I'll have to get some of that & give it a try. Thanks for the suggestion. I wonder if laying it in the sun would help too...I'll post back & let you know if the oxyclean works.

Nylon's and some other synthetics are naturally sort of gray. What makes them white is dye....white dye.
Bleach will remove this dye....what you need to do is get a laundry detergent that has "whiteners" in it and soak your shirt in a very strong solution of that detergent for about 12 to 20 hours.
rinse and dry.

I am so pleased! The Oxyclean worked. Three scoops in a sink of warm water for about an hour and my shirt looks like new again. That is some good stuff! Graywings, thank you :)

Lindac thanks also for your suggestion. I knew not to use bleach, but didn't think about the Detergent w/whiteners.

Anyway...problem solved and I'm so pleased...just Love that white shirt!