Those yellow underarm stains that appear on your white shirts are caused by a reaction between the aluminum in your deodorant and your sweat.   They can be really difficult to remove.

Here’s a simple pantry combination to try for white washable fabrics.

Soak the area in a mixture of equal parts water, baking soda, and peroxide (about 1/4 cup of each).  Gently scrub with a toothbrush to remove any particles then wash as usual.