Yellow underarm stains result from a reaction between the aluminum compounds found in antiperspirants and the salt found in sweat. If shirts are washable, there are several methods, including vinegar, that are used to remove these unsightly stains. The success of any of these methods depends on the shirt's fabric content and the age of the stain.
Ensure that the clothing item is washable
Refer to the care label to make sure the item is washable. If the care label states "dry clean only," don't attempt to wash the garment. Follow the instructions, and take the item to a dry cleaner.
Pour vinegar on the stain
Prior to washing, pour undiluted white vinegar directly onto the yellow underarm stain. Vinegar helps remove the stain and freshens up the item. It's safe to use vinegar on white and colored clothing.
Wash as usual
Let the vinegar sit on the stain for about 30 minutes, then wash the item in the usual manner. This method is best used on sturdy fabrics, such as a basic cotton shirt or dress.
Prevent the stain from returning
Keep yellow underarm stains away by switching to an aluminum-free deodorant and allowing deodorant to dry completely before dressing. If possible, try wearing an undershirt to protect more expensive outer shirts from underarm stains.