White shirts are a staple of every wardrobe, but yellow deodorant stains don't have to be. Many deodorants also contain antiperspirant ingredients like aluminum, which reacts with the salts in sweat to cause yellowing in white fabric. While ordinary detergents won't do much for armpit stains, fortunately there is a simple household product you can use to break down the proteins in stains to keep your white shirts looking like new.
Mix ¼ cup hydrogen peroxide with ¼ cup water in a small bowl. Thoroughly soak the yellow stains by pouring the mixture directly onto the shirt.
If it's a sunny day, hang the solution-soaked shirt outside on a clothes line or drape it over a towel-covered table. Leave the shirt in the sun for three to four hours. Periodically apply more solution if the shirt becomes dry. The sun will help evaporate the peroxide—and remove the aluminum buildup with it. If drying the shirt outside isn't possible, allow the soaked shirt to sit inside for at least 1 hour.
Rinse the shirt with cold water until all hydrogen peroxide has been removed. If the stain remains, repeat with a higher concentration of hydrogen peroxide.
Wash the white shirt on a cold cycle. Allow it to air dry.