Why are the armpits of my white shirts turing yellow?

eww.. haha jkjk it might be from sweat, because your body is constantly perspirating every minute (sweating) it could also be the type of deoderant your wearing... try using anti-pereperant to prevent them, and look for a deoderant that goes on clear... as for washing, maybe some kind of high-quality detergants might help =D