Perspiration and deodorant can result in some unpleasant dark underarm stains on your shirts. These stains can appear on many different types of garments, from dress shirts to undershirts. Being prepared to deal with dark underarm stains can make it less frustrating and alarming when you notice this type of stain on your clothes. Understand that even the darkest of underarm stains can be removed from most types of fabrics; therefore, a stain from perspiration, deodorant or both does not necessarily mean that your shirt is ruined.
Rinse out the shirt with plain water. This alone can often remove some of the stain.
Rub concentrated laundry detergent into the dark underarm stains. Let it sit for 30 minutes.
Place your shirt in the sun while the concentrated detergent soaks in. The sun is a natural stain remover.
Wash out the laundry detergent by running the shirt through the washing machine in cold water. Let it air-dry.
Mix together ½ cup of water and 1 tbsp. of white vinegar. Saturate the dark underarm stain in this mixture.
Let the vinegar solution sit on the stain for at least 20 minutes. Rinse and wash again with cold water.