Check the care tag on your underwear before you wash it to be sure it can be bleached. Fabrics such as silk, wool and spandex often cannot be bleached without damage to the garment.
Start a wash cycle. As the washing machine tub is filling with water, add 3/4 cup of chlorine bleach along with your regular laundry detergent. If your washer has a bleach dispenser, simply fill the dispenser before you start the wash cycle.
Place the white underwear into the washing machine, and allow the wash cycle to run its course. You may want to run an extra rinse cycle at the end to be sure all the bleach has been rinsed from the underwear. Any bleach left behind may weaken the fabric.
Remove the underwear from the washing machine at the end of the cycle, and dry it as usual.