1. Place a one or two drops (pasteur pipet) of alkaline bleach solution on a seeded regular (60 mm) plate, near the edge, not in the lawn. (Be sure to label the plate beforehand.)
2. From the contaminated plate, pick up 10-20 gravid adult hermaphrodites. (It helps to have lots of bacteria on your pick - then you can transfer many worms at one time.) Swish the worms into the drop of bleach on the new plate. They will die within 30 sec - 2 min. All the worms and most bacteria will die. Eggs survive and hatch by the next day, as long as the bleach solution is not too strong.
If you don't have a lot of worms in the drop, or you are in a hurry to get a good size culture going, you can transfer another "pickful" or two of worms to the bleach. The bleach will eventually soak into the plate.
If your plate is old enough that there are few or no gravid adult hermaphrodites, simply "chunk" the plate and wait a few days until you have plenty of adults.
3. The next day, pick L1 worms from the lawn to a new clean plate. They will be especially located at the lawn edge adjacent to where the drop was. Do not pick them from the region of the bleach drop. Picking worms the following day is essential to getting a clean culture, since some contamination inevitably survives the bleach. If necessary, you can transfer worms 2-3 days after bleaching, but the rate of successful cleaning drops as you wait longer.
If you bleach a sickly or severely Unc strain, it may be necessary to pick worms out of the drop location. This increases the risk of bringing along the contaminant. You can increase success by transferring these worms yet again to a new plate in a few hours or the next day.