The carcinogenicity of atmospheric pollutants from various urban sources in the U.S.A. has been demonstrated by the subcutaneous injection of trace quantities of organic extracts in newborn mice. The incidence of tumors, which included hepatomas, multiple pulmonary adenomas, and lymphomas, varied widely with the source of the pollutant. The results of these experiments indicate that, besides benzo[α] pyrene, other major classes of carcinogens are also present in polluted air. These experiments also indicate a high potential carcinogenic hazard of polluted air to man.