Mix 1/2 cup of table salt with 1/4 cup of hot water in a bowl. Dampen a cotton ball in the salt water and blot it over the antiperspirant stain until you remove it completely.
Dissolve any remaining deodorant stains by blotting them with white vinegar. Launder the t-shirt in the hottest water possible. Remove the t-shirt from the washer and examine it for the antiperspirant stain. Do not put the shirt in the dryer because it can cause a lingering deodorant stain to become permanent.
Apply ammonia to tough deodorant stains with a washcloth. Blot the stain instead of rubbing on it because you can cause the stain to spread. Rinse the ammonia from the t-shirt.
Treat the deodorant stain with a commercial pre-wash stain-removal solution. Read the directions on the pre-treatment carefully to prevent damage to the shirt.
Wash the shirt with the hottest water setting possible. Hang the t-shirt to dry and carefully examine it after it dries to ensure that the antiperspirant stain is completely gone.