This paper describes carburetor icing field tests performed on a large scale. In one controlled test, a base fuel and three anti-icing additives were compared for stalling tendencies by 120 private motorists operating their own personal cars over a four-month period. Results were analyzed using a digital computer and show that under the atmospheric conditions most conducive to icing (40°F. ambient and 100% relative humidity) stalling occurred with the base fuel in 43% of the observation periods. Additives cut this incidence by two-thirds. When stalling occurred, the average number of stalls per observation period was 1.7. Most cars stalled once or twice and only a few cars stalled as many as four times. Some makes of cars appeared more prone to stall than others, and more stalling was encountered with manual chokes than with automatic chokes.