The simplest way to remove underarm stains from shirts is to use a cleaning product like OxiClean or Raise Yellow Stain Remover. However, there are also household methods that use common items from the grocery store.
To use OxiClean or Raise Yellow Stain Remover, apply a strong mix of OxiClean or a liberal amount of the Raise Yellow stain remover, use a brush to rub it into the fabric and let it sit for 30 minutes. After 30 minutes, wash the clothes in a washing machine with hot water.
Another method is to mix an equal amount of baking soda, hydrogen peroxide and water. Rub the solution into the stain, and let it sit for 30 minutes.
Alternatively, create an equal mix of an oxygen-based bleach and ammonia. Rub the mix into the clothes, and let it sit for 30 minutes. Dark or colored clothes without a yellow stain may still have a stiffness at the underarm. Soak the clothing in vinegar, and then wash it with cold water. The stains are a result of the chemicals in sweat reacting to the aluminum in deodorant. If the stains persist or are more pronounced than usual, look into trying out a new deodorant brand that uses less aluminum.