L.J.
Hi A.,
I had that same problem with some of my shirts and pretty much all of my husband's shirts. When I was in college I took a textile class and I asked the teacher about it. She told me that the sweat actually created a chemical reaction within the cotton fibers that couldn't ever really be fixed. She said that not everyone has the type of sweat that does this, but that many people do. Wierd, huh? She did say that certain products could "mask" it though. I've tried adding OxyClean in with my whites and it helps a little, but the arm pits of the shirts never look as white as the rest of the shirt. And it seems like the next time my husband wears the shirt, it goes right back to looking yucky. Probably not the answer you wanted to hear, but maybe you all have the same sweat as my husband and I (what a thing to have in common!) Anyhow, good luck!