Water utilities and public-health agencies field-testing drinking water must wait about 72 hours to get lab results. Tox-Spot decreases the wait to 15 minutes. Using a briefcase-size portable laboratory kit, a field agent mixes a vial of bioluminescent bacteria with a milliliter of water sampled from any point along the water distribution network, from source to tap. Exposure to even trace amounts of toxins will measurably weaken the intensity of light the bacteria emit. Utilities could be using ToxSpot to test water supplies in the U.S. by next year.