Try putting it in the dishwasher on the top rack...it will sanitize it as well.
@Ally:
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A bath mat (that has done time with dirty feet and "other" funk) in your dishwasher? Where you wash the dishes you eat off of? You must really have a lot of faith in your dishwasher...either that, or it breathes fire when you're not looking.
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Throw it outside, spray it down with straight bleach on both sides, let it dry, rinse. I don't use those things, but that would be my best guess. But dishwasher...um...NO. 4951