Biossays performed on mice with four subcutaneously injected solvent fractions of the particulate phase of air pollutants of eight cities containing aromatic as well as aliphatic hydrocarbons, yielded cancerous reactions with all four fractions. The cancer yield obtained by the various fractions differed with the fraction as well as with the city from which the air-pollutants were collected. With the exception of air pollutants of Birmingham, which showed the highest total cancer yield and which had also the highest amount of 3.4-benzpyrene, there was no consistent correlation between the degree of carcionogenic potency of the solvent fractions bioassayed, the amount of 3.4-benzpyrene, and the reported lung cancer mortality rate in the eight cities studied. Every community has an atmospheric carcinogenic spectrum which depends upon industrial activities, traffic conditions, climate, solarization, and geologic conditions. An exploration of the relative role and importance of these factors for the lung cancer panorama of individual cities should include an investigation not only of the aromatic and aliphatic hydrocarbons present in the particulate phase of air pollutants but also of metallic, mineralic and radio-active constituents as well as the relative chemical stability and the physical status of some of these elements in the atmosphere.