This paper describes part of the joint effort by the General Motors Research Laboratories and the Sloan-Kettering Institute to evaluate the contribution by automobiles to the trace amounts of carcinogenic hydrocarbons in the atmosphere. Benzene-Soluble “tar”, containing carcinogenic polynuclear aromatic hydrocarbons, is recovered from the total volume of exhaust and blowby gases of an engine operated on a simulated city driving schedule.Fuel composition was found to influence both the rate of emission of “tar” and the concentration in the “tar” of benzo(a)pyrene, a carcinogen. Iso-octane and diisobutylene fuels gave only 20% as much “tar” as did a typical commercial gasoline, and a fuel comprised of equal volumes of ortho-xylene and benzene gave 70% as much “tar” as did gasoline. The emission rate of benzo(a)pyrene with diisobutylene fuel was only 7% of the rate with gasoline.Benzo(a)pyrene emission in blowby gas was less than 4% of the total benzo(a)pyrene emitted from the engine.On the assumption that benzo(a)pyrene from automobiles settles out of the air in the same manner as the lead from exhaust gas, automobiles may contribute, on the average, about 2% of the total benzo(a)pyrene in the air over cities. On the other hand, if benzo(a)pyrene is dispersed with little or no settling, as is the case for carbon monoxide from exhaust gas, the contribution of benzo(a)pyrene by automobiles may average about 10%.This paper describes part of the joint effort by the General Motors Research Laboratories and the Sloan-Kettering Institute to evaluate the contribution by automobiles to the trace amounts of carcinogenic hydrocarbons in the atmosphere. Benzene-Soluble “tar”, containing carcinogenic polynuclear aromatic hydrocarbons, is recovered from the total volume of exhaust and blowby gases of an engine operated on a simulated city driving schedule.Fuel composition was found to influence both the rate of emission of “tar” and the concentration in the “tar” of benzo(a)pyrene, a carcinogen. Iso-octane and diisobutylene fuels gave only 20% as much “tar” as did a typical commercial gasoline, and a fuel comprised of equal volumes of ortho-xylene and benzene gave 70% as much “tar” as did gasoline. The emission rate of benzo(a)pyrene with diisobutylene fuel was only 7% of the rate with gasoline.Benzo(a)pyrene emission in blowby gas was less than 4% of the total benzo(a)pyrene emitted from the engine.On the assumption that benzo(a)pyrene from automobiles settles out of the air in the same manner as the lead from exhaust gas, automobiles may contribute, on the average, about 2% of the total benzo(a)pyrene in the air over cities. On the other hand, if benzo(a)pyrene is dispersed with little or no settling, as is the case for carbon monoxide from exhaust gas, the contribution of benzo(a)pyrene by automobiles may average about 10%.