An autoradiographic technique is described for quantitative measurement of DNA repair synthesis in respiratory epithelial cells of various animals, including man, in short-term organ culture. The technique should be useful for analysing the mechanism of carcinogenesis under conditions that mimic those in vivo. The validity of this system was examined by testing the abilities of about 50 chemicals to induce unscheduled DNA synthesis in rat tracheal epithelium in culture. The results suggest that this system should be useful for screening possible lung carcinogens that are suspected of damaging DNA of the respiratory organs. Species differences in DNA repair levels were examined by treating tracheas from six species (mice, rats, hamsters, guinea-pigs, pigs and bovines) with three carcinogens. The carcinogen-induced DNA repair levels differed considerably from species to species. However, the relative efficiencies of DNA repair in the six species also differed with different carcinogens. A preliminary experiment showed that human bronchial epithelium responded well to chemical carcinogens; for example, it showed active DNA repair synthesis in response to some ultimate metabolites of benzo[a]pyrene.