Underarm stains on garments from the washing machine

This type of marking is caused by an underarm deodorant reacting with sweat, resulting in this type of hard to remove stain.  Obviously this does not happen with all deodorants and is probably a combination of the chemicals in your perspiration reacting with the deodorant, plus the material of the garment.

Removing the stains

You should soak the garment overnight in a solution of water and peroxide bleach, but ensure there is plenty of solution as you do not want to let the bleach dry out on your garment.

You can then rewash the garment on the normal cycle identified on the care label inside the garment.

You can also pre-soak the affected items or washing immediately after use, to avoid the perspiration drying out.  This is particularly a good idea with sports kits, as they are more likely to be marked in this way.

It may also be a good idea to try a different deodorant, as a different brand may not cause a reaction with the chemicals in your perspiration.