Are underarm stains caused by sweat or antiperspirant/deodorant? Who knows. Anyway, getting rid of those dreadful stains can be done with a homemade underarm stain remover: 1 part Dawn (or any dishwashing liquid, I assume), 2 parts hydrogen peroxide, then add baking soda to create a paste. Use brush to work it in, and let it sit for 1 hour. Then launder. Hope it works.