White armpit stains show up on your dark shirts as a result of perspiration or deodorant stains. The best way to deal with white armpit stains is to prevent them by using a good deodorant and wearing an undergarment under your nice shirt, if possible. But even when you take preventative measures, armpit stains occur on hot days or when you're nervous. Remove white armpit stains from dark shirts carefully so the dark shirt doesn't end up bleached or discolored.
Mix 1 tbsp. lemon juice and 1 tbsp. table salt in a small bowl. Your mixture should be like a thin paste.
Scrub the lemon and salt mixture into the white armpit stain on your dark shirt with an old toothbrush. Work the abrasive mixture into the fibers of the fabric. Allow it to sit for 15 minutes.
Flush your dark shirt with cold water to remove the lemon and salt paste and wash away the white armpit stain.
Apply an enzyme prewash stain remover to the site of any remaining stain. Use enough to cover the stain and rub it into the fabric and the stain with your fingers. Leave the prewash on the white armpit stain for 15 minutes.
Add 1 cup of white vinegar, along with laundry detergent, to the wash cycle of your laundry machine. The white vinegar will brighten the color of your dark shirt, helping restore color where the white armpit stains were removed.