MMT (methylcyclopentadienyl manganese tricarbonyl) is an antiknock additive for unleaded gasoline, which is now required for cars equipped with catalytic converters. Because of its effectiveness, MMT is economically attractive compared with achieving antiknock quality by refinery processing.Use of MMT in gasoline at a concentration of 0.125 g of contained manganese per gallon provides, on the average, about 2 road octane numbers. Compared to processing, this could represent a savings in crude oil of about 1%. Like other antiknocks, the economic attractiveness of MMT is greatest at low concentrations.Extensive road and dynamometer engine tests have shown that use of MMT in the recommended concentration range is compatible with general aspects of car operation--octane number requirement, exhaust valve and spark plug durability, and exhaust gas recycle for NOx control. Moreover, beneficial effects in exhaust valve guide and seat wear have been observed with MMT in some tests.In tests using cars operated on the 50,000-mile EPA certification schedule, the use of MMT had no significant adverse effect on emissions as compared to unleaded gasoline. Other tests have shown a decrease in exhausted polynuclear aromatics through the use of MMT.MMT does not lessen the effectiveness of exhaust catalysts in oxidizing unburned hydrocarbons and carbon monoxide. Some plugging of monolithic exhaust catalysts has occurred when cars on the road are operated under conditions subjecting the catalyst to severe stresses. However, rapid catalyst deterioration has been observed under these conditions whether or not manganese is present. No catalyst plugging has been observed with MMT in more typical driving conditions.The manganese in MMT is converted to Mn3O4 in the exhaust. We estimate that the median airborne manganese concentration in urban areas from widespread use of MMT would be 0.05 μg/m3; in freeway situations, estimated concentrations would be generally less than 1 μg/m3, rising above this level only under the most unfavorable meterological conditions.