Presently, the carcinogenicity of polluted air is determined by repeated skin painting or subcutaneous injection of crude organic extracts of atmospheric particulates in adult mice. In general such techniques are not ideal, inasmuch as large amounts of extracts are required to produce significant yields of local tumors, and these may take one to two years to develop. A photodynamic bioassay for poly cyclic atmospheric pollutants, based on the previous demonstration of a positive and significant association between high photodynamic activity and carcinogenicity in a large series of polycxjclics, has been developed in an attempt to provide a simple and rapid, but presumptive, indirect index of carcinogenicity attributable to polycyclic compounds. Photodynamic activity, measured with Paramecium caudatum, was found to vary widely between organic extracts, and even more widely between 6 eluted fractions thereof, from over 100 different sources in the U.S.A. Additionally, the carcinogenicity of organic extracts of atmospheric pollutants has been directly measured with newborn mice, based on the well-known enhanced response cf neonates to a variety of defined chemical carcinogens. Mice were injected subcutaneously with extracts at doses of 55 mg or less, fractionally administered in the first 21 days of life. A high incidence of pulmonary adenomas, and hepatomas, and a lower incidence of lymphomas developed over a period from 12 to 50 weeks. Current correlative evaluation with chemical and other data indicates the practical utility of both assays for carcinogenic air pollutants.