So, I love white shirts. I don't love what happens to white shirts in the armpits though. I've tried various remedies, but this was the first that worked and it was so easy that I feel compelled to share.
a gallon or 2 of hot hot water (our tap gets pretty hot, so I just used that, but you could boil some up I supposed)
1 cup bleach
1 cup dishwasher detergent (I used the gel kind from Walmart)
Mix it all up, through in your clothes, make sure they are all covered. Let sit for at least an hour. Mine soaked for 2 or 3. Then wring them out and through them in the washer. Dishwasher soap is specially formulated not to suds up so it's safe to put in your washer.
Seriously, I don't have stains on shirts that I've had for years now. It even got poop stains out of Ellie's white onsies. YEAH!