Those stains are mineral deposits...deposits of Aluminium chlorohydrate + minerals from perspiration.

You can get rid of them with white vinegar.

Pour undiluted white vinegar over the underarm areas. Allow to sit for about 20 minutes. Then launder as usual.

The acids in the Vinegar will break down the deposits, allowing your washer to wash them away. Plus, Vinegar is safe for all colors.

Don't use bleach on them....bleach makes those stains worse.

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