Scrape off as much peanut butter as you can with a spoon.
Apply a pre-wash stain remover or dishwashing liquid specifically made to break up grease directly on the stain.
Allow the stain remover to sit for 30 minutes.
Place soiled clothing in the washing machine, alone or with a full load.
Wash on the warmest setting that is acceptable for the fabric.
Check the stain when the washer stops. If the stain remains, re-apply the stain remover or dishwashing liquid and wash in warm water again. Do not place in the dryer until the stain is removed.