Ethyl has been conducting extensive emission studies of two car fleets to investigate any effect of MMT on exhaust emissions from production cars. One fleet is being operated by Ethyl for 50,000 miles on an EPA-type durability route. This fleet consists of thirty 1977 California cars - six cars each of five models (four U.S. and one import). Two of each model are using clear certification fuel, with two each using the certification fuel plus 1/32 g Mn/gallon or 1/16 g Mn/gallon. Although the test is still running, the 16 U.S. cars using clear fuel or 1/16 g Mn/gallon have completed the 50,000 miles. For these cars, the average increase in HC, CO, and NOx emissions was essentially the same for both the clear-fuel cars and the MMT cars, with deterioration rates being somewhat lower for the MMT cars.The second test involves 20 cars from a large oil company fleet of 1975 and 1976 catalyst-equipped cars used by salesmen in the field. Ten of the cars have used clear fuel and ten have used fuel containing 1/8 g Mn/gallon, with average mileage exceeding 50,000 miles. Engine-out emission data were obtained before and after deposit removal, and tailpipe emissions were measured as received and after tune-up. The engine-out data show only little differences in stabilized emission levels between the clear-fuel cars and the MMT cars, both before and after deposit removal. As-received tailpipe emission data showed a larger spread in individual car ratings, probably because of missing air-pump belts on some cars and other malfunctions of the air delivery system. Average tailpipe emissions after tune-up showed only small differences between the clear-fuel and MMT cars, with the difference in hydrocarbon emissions resulting from the abnormally high HC emissions of one MMT car.