Yellow armpit stains can be embarrassing. Getting rid of them requires more than a simple wash in a machine. It often takes special deep-cleaning solutions and a lot of patience.
Apply deep-cleaning solution
Rub liquid cleaning solution or powder mixed with water directly onto the stains. Many companies offer products promising to remove armpit stains. "The Art of Manliness" tested several conventional solutions and discovered that OxiClean and Raise were the best at removing armpit stains.
Soak the shirt in a bucket with the solution for several hours or even overnight.
Run the shirt through a normal wash cycle. It is best not to put other clothes in the wash to avoid colors running. After the wash cycle, remove the shirt and dry it normally.